Fix resolution of local branch name in scm and rebaseline tools.

Tools/Scripts/webkitpy/common/checkout/scm/git.py

 # Copyright (c) 2009, 2010, 2011 Google Inc. All rights reserved.
 # Copyright (c) 2009 Apple Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 116 matching lines @@
         return self._run_git(["mv", "-f", origin, destination])
 
     def exists(self, path):
         return_code = self._run_git(["show", "HEAD:%s" % path], return_exit_code=True, decode_output=False)
         return return_code != self.ERROR_FILE_IS_MISSING
 
     def _branch_from_ref(self, ref):
         return ref.replace('refs/heads/', '')
 
     def current_branch(self):
-        return self._branch_from_ref(self._run_git(['symbolic-ref', '-q', 'HEAD']).strip())
+        ref = self._run_git(['rev-parse', '--symbolic-full-name', 'HEAD']).strip()
+        # Return an empty string if HEAD is detached.
+        return self._branch_from_ref('' if ref == 'HEAD' else ref)
 
     def _upstream_branch(self):
         current_branch = self.current_branch()
         return self._branch_from_ref(self.read_git_config('branch.%s.merge' % current_branch, cwd=self.checkout_root, executive=self._executive).strip())
 
     def _merge_base(self, git_commit=None):
         if git_commit:
             # Rewrite UPSTREAM to the upstream branch
             if 'UPSTREAM' in git_commit:
                 upstream = self._upstream_branch()
@@ skipping 163 matching lines @@
         if self.current_branch() != self._branch_tracking_remote_master():
             return False
         if len(self._local_commits(self._branch_tracking_remote_master())) > 0:
             return False
         return True
 
     def ensure_cleanly_tracking_remote_master(self):
         self._discard_working_directory_changes()
         self._run_git(['checkout', '-q', self._branch_tracking_remote_master()])
         self._discard_local_commits()