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>

subcmds/init.py

@@ -96,7 +96,7 @@ to update the working directory files.
     g.add_option('-b', '--manifest-branch',
                  dest='manifest_branch',
                  help='manifest branch or revision', metavar='REVISION')
-    g.add_option('--current-branch',
+    g.add_option('-c', '--current-branch',
                  dest='current_branch_only', action='store_true',
                  help='fetch only current manifest branch from server')
     g.add_option('-m', '--manifest-name',