https+auth: use a ~/.git-credentials file if it exists to fetch credentials
In some situations we code the plain auth-less uri in the hgrc paths section (for convenience) and want to use a centrally-stored credentials file.
Though it is possible to use the hgrc [auth] section, it would be also nice to access .git-credentials (still usefull for git direct access), avoiding double maintenance of credentials.
This probably does not cover all possible situations with git credentials handling but it provides a working base.
closes #305 (closed)
(created on behalf of @aurelien.campeas who asked me to take care of that one)