Allow pattern-based tagging from remote branches

[totten]

We use git scan branch and git scan tag for pre-releases and release-candidates in Civi ( http://wiki.civicrm.org/confluence/display/CRM/Release+checklist ), e.g.

git scan tag 4.7.10-pre1 master -p -n -T

This automatically detects branches like civicrm-drupal's 7.x-master. However, if you haven't locally checked out 6.x-master or 8.x-master, then it won't find them. (Those are available as remote branches... not as local branches.)

For this use-case, the intent is really to produce tags/branches based off the upstream code, e.g.

git scan tag 4.7.10-pre1 origin/master -p -n -T

I don't think git scan tag or git scan branch would handle that correctly right now.