Evidence indicates that running non-builtin git commands is very
slow in msysgit, slow enough to dominate the running time of
gclient sync.  With this change, gclient never shells out to
git-cache; it import the lib directly instead.

R=agable@chromium.org,hinoka@chromium.org
BUG=

Review URL: https://codereview.chromium.org/229653002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@262759 0039d316-1c4b-4281-b951-d872f2087c98