Created the 'GitRefish' class to centralize 'git' refish parsing and consistent
usage by 'gclient' 'git' code.

BUG=373504
TEST=localtest
R=agable@chromium.org, iannucci@chromium.org

Review URL: https://codereview.chromium.org/328843005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@281553 0039d316-1c4b-4281-b951-d872f2087c98