Run prune in git cache update bootstrap to remove loose objects

BUG=411236

Review URL: https://codereview.chromium.org/545213002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@291865 0039d316-1c4b-4281-b951-d872f2087c98

git_cache.py

@@ -424,7 +424,8 @@ class Mirror(object):
     # The files are named <git number>.zip
     gen_number = subprocess.check_output(
         [self.git_exe, 'number', 'master'], cwd=self.mirror_path).strip()
-    self.RunGit(['gc'])  # Run Garbage Collect to compress packfile.
+    # Run Garbage Collect to compress packfile.
+    self.RunGit(['gc', '--prune=all'])
     # Creating a temp file and then deleting it ensures we can use this name.
     _, tmp_zipfile = tempfile.mkstemp(suffix='.zip')
     os.remove(tmp_zipfile)