Reland of Use gclient's git_cache setting in bot_update/api.py

https://codereview.chromium.org/1762513002/

Use gclient's git_cache setting in bot_update/api.py

BUG=589140
R=szager@chromium.org,iannucci@chromium.org

Review URL: https://codereview.chromium.org/1784043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@299260 0039d316-1c4b-4281-b951-d872f2087c98

recipe_modules/bot_update/api.py

@@ -143,7 +143,7 @@ class BotUpdateApi(recipe_api.RecipeApi):
         ['--spec', spec_string],
         ['--root', root],
         ['--revision_mapping_file', self.m.json.input(rev_map)],
-        ['--git-cache-dir', self.m.path['git_cache']],
+        ['--git-cache-dir', cfg.cache_dir],
 
         # 3. How to find the patch, if any (issue/patchset/patch_url).
         ['--issue', issue],