fix new "var never mutated" error on Zig 0.12.0-dev.1663+6b1a823b2

Fixes these errors (introduced in ziglang/zig#18017
and ziglang/zig@6b1a823 ):

```
src/main.zig:290:13: error: local variable is never mutated
        var line_ = line_fbs.getWritten();
            ^~~~~
src/main.zig:290:13: note: consider using 'const'
src/main.zig:450:17: error: local variable is never mutated
            var path = std.fs.path.joinZ(allocator, &.{p, "ncdu", "config"}) catch unreachable;
                ^~~~
src/main.zig:450:17: note: consider using 'const'

...
```

Will be included in future Zig 0.12, this fix is backward compatible:
ncdu still builds and runs fine on Zig 0.11.0.

Signed-off-by: Eric Joldasov <bratishkaerik@getgoogleoff.me>

src/browser.zig

@@ -349,8 +349,8 @@ const info = struct {
     var links_idx: usize = 0;
 
     fn lt(_: void, a: *model.Link, b: *model.Link) bool {
-        var pa = a.path(false);
-        var pb = b.path(false);
+        const pa = a.path(false);
+        const pb = b.path(false);
         defer main.allocator.free(pa);
         defer main.allocator.free(pb);
         return std.mem.lessThan(u8, pa, pb);

src/exclude.zig

@@ -56,7 +56,7 @@ const Pattern = struct {
 
     fn parse(pat_: []const u8) *const Pattern {
         var pat = std.mem.trimLeft(u8, pat_, "/");
-        var top = main.allocator.create(Pattern) catch unreachable;
+        const top = main.allocator.create(Pattern) catch unreachable;
         var tail = top;
         tail.sub = null;
         while (std.mem.indexOfScalar(u8, pat, '/')) |idx| {
@@ -147,7 +147,7 @@ fn PatternList(comptime withsub: bool) type {
         fn append(self: *Self, pat: *const Pattern) void {
             std.debug.assert((pat.sub != null) == withsub);
             if (pat.isliteral) {
-                var e = self.literals.getOrPut(main.allocator, pat) catch unreachable;
+                const e = self.literals.getOrPut(main.allocator, pat) catch unreachable;
                 if (!e.found_existing) {
                     e.key_ptr.* = pat;
                     e.value_ptr.* = if (withsub) .{} else {};
@@ -250,16 +250,16 @@ pub fn getPatterns(path_: []const u8) Patterns {
     var pat = root;
     defer pat.deinit();
     while (std.mem.indexOfScalar(u8, path, '/')) |idx| {
-        var name = main.allocator.dupeZ(u8, path[0..idx]) catch unreachable;
+        const name = main.allocator.dupeZ(u8, path[0..idx]) catch unreachable;
         defer main.allocator.free(name);
         path = path[idx+1..];
 
-        var sub = pat.enter(name);
+        const sub = pat.enter(name);
         pat.deinit();
         pat = sub;
     }
 
-    var name = main.allocator.dupeZ(u8, path) catch unreachable;
+    const name = main.allocator.dupeZ(u8, path) catch unreachable;
     defer main.allocator.free(name);
     return pat.enter(name);
 }

src/main.zig

@@ -287,7 +287,7 @@ fn tryReadArgsFile(path: [:0]const u8) void {
             error.EndOfStream => if (line_fbs.getPos() catch unreachable == 0) break,
             else => |e| ui.die("Error reading from {s}: {s}\nRun with --ignore-config to skip reading config files.\n", .{ path, ui.errorString(e) }),
         };
-        var line_ = line_fbs.getWritten();
+        const line_ = line_fbs.getWritten();
 
         var line = std.mem.trim(u8, line_, &std.ascii.whitespace);
         if (line.len == 0 or line[0] == '#') continue;
@@ -447,11 +447,11 @@ pub fn main() void {
         tryReadArgsFile("/etc/ncdu.conf");
 
         if (std.os.getenvZ("XDG_CONFIG_HOME")) |p| {
-            var path = std.fs.path.joinZ(allocator, &.{p, "ncdu", "config"}) catch unreachable;
+            const path = std.fs.path.joinZ(allocator, &.{p, "ncdu", "config"}) catch unreachable;
             defer allocator.free(path);
             tryReadArgsFile(path);
         } else if (std.os.getenvZ("HOME")) |p| {
-            var path = std.fs.path.joinZ(allocator, &.{p, ".config", "ncdu", "config"}) catch unreachable;
+            const path = std.fs.path.joinZ(allocator, &.{p, ".config", "ncdu", "config"}) catch unreachable;
             defer allocator.free(path);
             tryReadArgsFile(path);
         }
@@ -462,7 +462,7 @@ pub fn main() void {
     var export_file: ?[:0]const u8 = null;
     var quit_after_scan = false;
     {
-        var arglist = std.process.argsAlloc(allocator) catch unreachable;
+        const arglist = std.process.argsAlloc(allocator) catch unreachable;
         defer std.process.argsFree(allocator, arglist);
         var args = Args.init(arglist);
         _ = args.next(); // program name
@@ -506,7 +506,7 @@ pub fn main() void {
     event_delay_timer = std.time.Timer.start() catch unreachable;
     defer ui.deinit();
 
-    var out_file = if (export_file) |f| (
+    const out_file = if (export_file) |f| (
         if (std.mem.eql(u8, f, "-")) stdout
         else std.fs.cwd().createFileZ(f, .{})
              catch |e| ui.die("Error opening export file: {s}.\n", .{ui.errorString(e)})
@@ -573,7 +573,7 @@ pub fn handleEvent(block: bool, force_draw: bool) void {
 
     var firstblock = block;
     while (true) {
-        var ch = ui.getch(firstblock);
+        const ch = ui.getch(firstblock);
         if (ch == 0) return;
         if (ch == -1) return handleEvent(firstblock, true);
         switch (state) {

src/model.zig

@@ -319,7 +319,7 @@ pub const devices = struct {
     var lookup = std.AutoHashMap(u64, DevId).init(main.allocator);
 
     pub fn getId(dev: u64) DevId {
-        var d = lookup.getOrPut(dev) catch unreachable;
+        const d = lookup.getOrPut(dev) catch unreachable;
         if (!d.found_existing) {
             d.value_ptr.* = @as(DevId, @intCast(list.items.len));
             list.append(dev) catch unreachable;
@@ -400,7 +400,7 @@ pub const inodes = struct {
                 nlink += 1;
                 var parent: ?*Dir = it.parent;
                 while (parent) |p| : (parent = p.parent) {
-                    var de = dirs.getOrPut(p) catch unreachable;
+                    const de = dirs.getOrPut(p) catch unreachable;
                     if (de.found_existing) de.value_ptr.* += 1
                     else de.value_ptr.* = 1;
                 }

src/scan.zig

@@ -293,7 +293,7 @@ const Context = struct {
         wr.print("{d}", .{std.time.timestamp()}) catch |e| writeErr(e);
         wr.writeByte('}') catch |e| writeErr(e);
 
-        var self = main.allocator.create(Self) catch unreachable;
+        const self = main.allocator.create(Self) catch unreachable;
         self.* = .{ .wr = buf };
         return self;
     }
@@ -555,7 +555,7 @@ pub fn setupRefresh(parent: *model.Dir) void {
 // To be called after setupRefresh() (or from scanRoot())
 pub fn scan() void {
     defer active_context.deinit();
-    var dir_ = std.fs.cwd().openDirZ(active_context.pathZ(), .{}, true) catch |e| {
+    const dir_ = std.fs.cwd().openDirZ(active_context.pathZ(), .{}, true) catch |e| {
         active_context.last_error = main.allocator.dupeZ(u8, active_context.path.items) catch unreachable;
         active_context.fatal_error = e;
         while (main.state == .refresh or main.state == .scan)

src/ui.zig

@@ -342,9 +342,9 @@ pub fn init() void {
     if (inited) return;
     clearScr();
     if (main.config.nc_tty) {
-        var tty = c.fopen("/dev/tty", "r+");
+        const tty = c.fopen("/dev/tty", "r+");
         if (tty == null) die("Error opening /dev/tty: {s}.\n", .{ c.strerror(@intFromEnum(std.c.getErrno(-1))) });
-        var term = c.newterm(null, tty, tty);
+        const term = c.newterm(null, tty, tty);
         if (term == null) die("Error initializing ncurses.\n", .{});
         _ = c.set_term(term);
     } else {