Revert of Consolidated 'git' refish parsing into a class (https://codereview.chromium.org/328843005/)

Reason for revert:
https://code.google.com/p/chromium/issues/detail?id=391871

Original issue's description:
> Consolidated 'git' refish parsing into a class
>
> 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
>
> Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=281553

TBR=dnj
BUG=391871

Review URL: https://codereview.chromium.org/370393002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@281572 0039d316-1c4b-4281-b951-d872f2087c98