help: unify command display

No functional changes, just unifying duplicate code paths.

Change-Id: I6afa797ca1e1eb90abdc0236325003ae070cbfb3
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/247293
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>