* Use base chrome svn url for reverts to work with
branches and tools repos.
* Add heuristic to drover so it can search multiple
svn repos for the likely revision to revert.

Each of chrome, chrome-internal, native_client are separated by
more than 20k revisions, so we can programmatically determine
which repo the user wants to revert with high confidence.

TESTS=
- pylint (part of presubmit)
- tried recent tools/build chrome revision
- tried NaCl revision
- tried NaCl revision with --no-alt-urls (got rev from chrome)
- tried NaCl without valid svn credentials (got rev from chrome)

Review URL: https://chromiumcodereview.appspot.com/12670008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@188792 0039d316-1c4b-4281-b951-d872f2087c98