subcmds: delete redundant dest= settings

Add a test to enforce this too.

Change-Id: I80b5cf567aa33db9c24b53428c66d69f9c1d8d74
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/478481
Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Scott Lee <ddoman@google.com>

subcmds/overview.py

@@ -37,7 +37,6 @@ are displayed.
         p.add_option(
             "-c",
             "--current-branch",
-            dest="current_branch",
             action="store_true",
             help="consider only checked out branches",
         )