depgraph: Do not trigger rebuilds for some options

Do not trigger rebuilds for options which do not merge packages
like --buildpkgonly, --fetchonly, and --fetch-all-uri.

Bug: https://bugs.gentoo.org/927301
Signed-off-by: Zac Medico <zmedico@gentoo.org>

lib/_emerge/depgraph.py

@@ -109,6 +109,8 @@
     import _emerge.stdout_spinner.stdout_spinner
 
 
+_NO_MERGE_OPTS = ("--buildpkgonly", "--fetchonly", "--fetch-all-uri")
+
 # Exposes a depgraph interface to dep_check.
 _dep_check_graph_interface = collections.namedtuple(
     "_dep_check_graph_interface",
@@ -1942,7 +1944,9 @@ def _process_slot_conflicts(self):
         for conflict in self._dynamic_config._package_tracker.slot_conflicts():
             self._process_slot_conflict(conflict)
 
-        if self._dynamic_config._allow_backtracking:
+        if self._dynamic_config._allow_backtracking and not any(
+            opt in self._frozen_config.myopts for opt in _NO_MERGE_OPTS
+        ):
             self._slot_operator_trigger_reinstalls()
 
     def _process_slot_conflict(self, conflict):
@@ -8967,7 +8971,7 @@ def _accept_blocker_conflicts(self):
         if self._dynamic_config._allow_backtracking:
             return False
         acceptable = False
-        for x in ("--buildpkgonly", "--fetchonly", "--fetch-all-uri", "--nodeps"):
+        for x in _NO_MERGE_OPTS + ("--nodeps",):
             if x in self._frozen_config.myopts:
                 acceptable = True
                 break