Git context menu (#24844)

Adds the non-entry specific right click menu to the panel, and the
features contained therin:

* Stage all
* Discard Tracked Changes
* Trash Untracked Files

Also changes the naming from "Changes"/"New" to better match Git's
terminology (though not convinced on this, it was awkward to describe
"Discard Changes" without a way to distinguish between the changes and
the files containing them).

Release Notes:

- N/A

crates/git/src/git.rs

@@ -37,7 +37,8 @@ actions!(
         // editor::RevertSelectedHunks
         StageAll,
         UnstageAll,
-        RevertAll,
+        DiscardTrackedChanges,
+        TrashUntrackedFiles,
         Uncommit,
         Commit,
         ClearCommitMessage