* Add revinfo as part of SCMWrapper interface.
* Implement revinfo for git and svn.
* Add unit tests.

Review URL: http://codereview.chromium.org/362008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@31280 0039d316-1c4b-4281-b951-d872f2087c98