Get rid of error in legacy git pillar when using branch mapping notation

This doesn't fix the problem of the branch not being mapped to the
specified branch, and this code is not going to be maintained going
forward (in favor of the gitfs-backed git_pillar code), but this will at
least prevent the branch from failing to checkout.

salt/pillar/git_pillar.py

@@ -316,6 +316,8 @@ class _LegacyGitPillar(object):
             branch = opts.get('environment') or 'base'
             if branch == 'base':
                 branch = opts.get('gitfs_base') or 'master'
+        elif ':' in branch:
+            branch = branch.split(':', 1)[0]
         return branch
 
     def update(self):