Bug 1104302 - dxr-build fails when the git's origin has been cloned from an http(s) address 

See bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1104302

> Note: let's discuss this PR, I'm sure there are better things to do than what I just did !
We should probably test all the way to the colon, i.e. `startswith('git:')`, in case there's someday a scheme called "gitorious", for instance.