Instead of having custom logic for dealing with cache directories, use
git_cache.py to populate caches.

Also fixes a bug in git_cache.py where it was looking for lockfiles in cwd rather than the cache dir.

Other changes:
* _Run now returns output.
* Always print to stdout in CheckCallAndFilterOutput, even if it gets a carriage return.  This is done because git progress report are carriage returns and not newlines and we don't want everything on the same line and not strip out the CRs.
* Removed members changed tests, its not very useful to know a new import is added.

BUG=339171

Review URL: https://codereview.chromium.org/180243006

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@254248 0039d316-1c4b-4281-b951-d872f2087c98