Fix git cache unlock invocation to specify cache dir.

TBR=cmp@chromium.org,agable@chromium.org
BUG=

Review URL: https://codereview.chromium.org/498043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291591 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -1223,7 +1223,8 @@ want to set 'managed': False in .gclient.
       cache_dir = os.path.abspath(cache_dir)
       # If running on a bot, force break any stale git cache locks.
       if os.environ.get('CHROME_HEADLESS'):
-        subprocess2.check_call(['git', 'cache', 'unlock', '--force', '--all'])
+        subprocess2.check_call(['git', 'cache', 'unlock', '--cache-dir',
+                                cache_dir, '--force', '--all'])
     gclient_scm.GitWrapper.cache_dir = cache_dir
     git_cache.Mirror.SetCachePath(cache_dir)