p4: respect init.defaultBranch

In `git p4 clone`, we hard-code the branch name `master` instead of
looking what the _actual_ initial branch name is. Let's fix that.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -4186,7 +4186,7 @@ class P4Clone(P4Sync):
 
         # create a master branch and check out a work tree
         if gitBranchExists(self.branch):
-            system([ "git", "branch", "master", self.branch ])
+            system([ "git", "branch", currentGitBranch(), self.branch ])
             if not self.cloneBare:
                 system([ "git", "checkout", "-f" ])
         else: