main/repo: add support for subcommand aliases

This supports [alias] sections with repo subcommands just like git.

Change-Id: Ie9235b5d4449414e6a745814f0110bd6af74ea93
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/255833
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>