## Context

I noticed that the project panel `select_next_git_entry` wasn't behaving
correctly. Turns out it was searching in reverse, which caused the
action to select itself or the last entry.

This PR corrects the behavior and adds a unit test that should stop
regressions.

Note: Since select next/prev git entry uses the same function as select
next/prev diagnostic, the test partially works for that as well.

Release Notes:

- Fix bug where `select_next_git_entry` project panel action would only
select a previous entry or the currently selected entry.

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Co-authored-by: Mikayla Maki <mikayla@zed.dev>