cleanup: use new dict & set generator styles

Change-Id: Ie34ac33ada7855945c77238da3ce644f8a9f8306
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/390374
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>

command.py

@@ -290,7 +290,7 @@ class Command:
                 output.end()
 
     def _ResetPathToProjectMap(self, projects):
-        self._by_path = dict((p.worktree, p) for p in projects)
+        self._by_path = {p.worktree: p for p in projects}
 
     def _UpdatePathToProjectMap(self, project):
         self._by_path[project.worktree] = project