Merge branch 'ps/p4-use-ref-api'

"git p4" update to prepare for reftable

* ps/p4-use-ref-api:
  git-p4: stop reaching into the refdb

Author: gitster

git-p4.py

@@ -4251,7 +4251,8 @@ def run(self, args):
         if self.tempBranches != []:
             for branch in self.tempBranches:
                 read_pipe(["git", "update-ref", "-d", branch])
-            os.rmdir(os.path.join(os.environ.get("GIT_DIR", ".git"), self.tempBranchLocation))
+            if len(read_pipe(["git", "for-each-ref", self.tempBranchLocation])) > 0:
+                   die("There are unexpected temporary branches")
 
         # Create a symbolic ref p4/HEAD pointing to p4/<branch> to allow
         # a convenient shortcut refname "p4".