prevent tracking git branch with detached head

tests/samples/projects/git-with-detached-head/dot_git/HEAD

@@ -0,0 +1 @@
+f4f242d698fa07c298592a66d6546ac9b6b34d1e

tests/samples/projects/git-with-detached-head/dot_git/config

@@ -0,0 +1,7 @@
+[core]
+	repositoryformatversion = 0
+	filemode = true
+	bare = false
+	logallrefupdates = true
+	ignorecase = true
+	precomposeunicode = true

tests/test_project.py

@@ -166,6 +166,26 @@ class LanguagesTestCase(utils.TestCase):
         self.assertEquals('git', self.patched['wakatime.offlinequeue.Queue.push'].call_args[0][0]['project'])
         self.assertEquals('master', self.patched['wakatime.offlinequeue.Queue.push'].call_args[0][0].get('branch'))
 
+    def test_git_detached_head_not_used_as_branch(self):
+        response = Response()
+        response.status_code = 0
+        self.patched['wakatime.packages.requests.adapters.HTTPAdapter.send'].return_value = response
+
+        tempdir = tempfile.mkdtemp()
+        shutil.copytree('tests/samples/projects/git-with-detached-head', os.path.join(tempdir, 'git'))
+        shutil.move(os.path.join(tempdir, 'git', 'dot_git'), os.path.join(tempdir, 'git', '.git'))
+
+        now = u(int(time.time()))
+        entity = os.path.join(tempdir, 'git', 'emptyfile.txt')
+        config = 'tests/samples/configs/good_config.cfg'
+
+        args = ['--file', entity, '--config', config, '--time', now]
+
+        execute(args)
+
+        self.assertEquals('git', self.patched['wakatime.offlinequeue.Queue.push'].call_args[0][0]['project'])
+        self.assertNotIn('branch', self.patched['wakatime.offlinequeue.Queue.push'].call_args[0][0])
+
     def test_svn_project_detected(self):
         response = Response()
         response.status_code = 0

wakatime/projects/git.py

@@ -38,11 +38,11 @@ class Git(BaseProject):
             head = os.path.join(self._project_base(), '.git', 'HEAD')
             try:
                 with open(head, 'r', encoding='utf-8') as fh:
-                    return u(fh.readline().strip().rsplit('/', 1)[-1])
+                    return self._get_branch_from_head_file(fh.readline())
             except UnicodeDecodeError:  # pragma: nocover
                 try:
                     with open(head, 'r', encoding=sys.getfilesystemencoding()) as fh:
-                        return u(fh.readline().strip().rsplit('/', 1)[-1])
+                        return self._get_branch_from_head_file(fh.readline())
                 except:
                     log.traceback('warn')
             except IOError:  # pragma: nocover
@@ -64,3 +64,8 @@ class Git(BaseProject):
         if split_path[1] == '':
             return None
         return self._find_git_config_file(split_path[0])
+
+    def _get_branch_from_head_file(self, line):
+        if u(line.strip()).startswith('ref: '):
+            return u(line.strip().rsplit('/', 1)[-1])
+        return None