ability to use the .git-credentials file if it exists

To operate with https + token one has to transmit things through the uri. For obvious reasons, separating the uri from the creds is a good idea.

In the mercurial world we can of course use the [auth] section (though as per #109 (closed) it was not always easy).

However even nicer would be the ability to handle ~/.git-credentials directly, avoiding an unnecessary redundancy.

(I have a small patch to do this)