'git cl format' became very slow (7
seconds) after the Blink merge. 'git cl upload' is also slow when
the presubmit requires formatting checks.

This CL speeds up 'git cl format' by reducing the number of calls to
'git diff' and by removing the glob parameter. The glob
parameter adds a couple of seconds as it scans the entire
repository.

Before: 'git cl format' takes 7 seconds
After: 'git cl format' takes 1.25 seconds

BUG=577647

Review URL: https://codereview.chromium.org/1585923004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298323 0039d316-1c4b-4281-b951-d872f2087c98