Currently, revision numbers are stored as int. This won't work
for git's SHA1s. So storing revisions as strings.

Review URL: http://codereview.chromium.org/208060

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26940 0039d316-1c4b-4281-b951-d872f2087c98