Add a lot of checking for stdout and fix one mis-assumption.
Make the code much more compact.
Make the git_hashes 'revisions' 1-based for consistency.

Review URL: http://codereview.chromium.org/2594001

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@48925 0039d316-1c4b-4281-b951-d872f2087c98