subrepo: clone of git sub-repository creates incorrect git branch (issue3870)

Mercurial handles git subrepos by incorrect way.
If the mercurial repo has a git sub-repo and somebody started
a new branch in the subrepo and push it into git, the next one
who will clone the whole repo will get incorrect branch name in the git
subrepo.

mercurial/subrepo.py

@@ -1266,7 +1266,7 b' class gitsubrepo(abstractsubrepo):'
 
         if remote not in tracking:
             # create a new local tracking branch
-            local = remote.split('/', 2)[2]
+            local = remote.split('/', 3)[3]
             checkout(['-b', local, remote])
         elif self._gitisancestor(branch2rev[tracking[remote]], remote):
             # When updating to a tracked remote branch,