init: Add '-c' as an alias to '--current-branch'

This makes it consistent with the short option for current-branch in
repo sync.

Change-Id: I2848e87f45a66ef8d829576d0c0c4c0f7a8636a0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/241700


Tested-by: Diogo Ferreira <deovferreira@gmail.com>
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>