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/diff.py

@@ -35,7 +35,6 @@ to the Unix 'patch' command.
         p.add_option(
             "-u",
             "--absolute",
-            dest="absolute",
             action="store_true",
             help="paths are relative to the repository root",
         )