useNavigate: Get pathname from useLocation instead of RouteContext

[chaance]

In #7880 it looks like useSearchParams, when called from a higher level route that renders an outlet, potentially directs to the wrong pathname when setSearchParams is called and a user is actually at a nested route.

This is actually a problem with useNavigate, I believe. When the resulting navigate function is called without an explicit pathname, we should stick with the current pathname returned by useLocation instead of the pathname constructed in the RouteContext. This ensures that navigate always directs users to a location with the same pathname they are currently on while only modifying either the hash or search depending on what is passed to navigate. Because setSearchParams abstracts navigate, this should fix #7880 and make useNavigate more stable in general.

[mjackson]

This looks great 👍 Can we also get some tests in place around this? I'm thinking we should have a test for navigate() with only a search string, and another one for only a hash. Both should make sure we don't manipulate the pathname in the navigation.