This allows having a git repo cloned on a specific branch or commit, similar to
what's possible for svn solutions by specifying a url like svnrepo/branches/foo

Having the possibility to check the code out on a specific branch makes the
initial code checkout workflow simpler, and allows specifying all the relevant
info directly in the fetch recipe, instead of  having to deal with branches
manually afterwards (which is the whole purpose of recipes in the first place)

So with this change, I'm able to do the following in my custom fetch recipe:

    solution = {
        'name' : 'src',
        'url' : 'git@github.snei.sony.com:SNEI/chromium.git@refs/heads/custombranch',
        <following lines skipped>

Review URL: https://codereview.chromium.org/289863012

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@273675 0039d316-1c4b-4281-b951-d872f2087c98