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/abandon.py

@@ -48,7 +48,6 @@ It is equivalent to "git branch -D <branchname>".
     def _Options(self, p):
         p.add_option(
             "--all",
-            dest="all",
             action="store_true",
             help="delete all branches in all projects",
         )