[GSoC] Move ~/.git-credential-cache to ~/.cache/git

I started working on this microproject and am not quite sure what is necessary for backwards compatibility. Since the socket is recreated whenever the credential daemon exits backwards compatibility shouldn’t really be a concern with regard to where the socket is located in the filesystem.

However, contrib/persistent-https depends on the socket being at ~/.git-credential-cache/socket, so changing the default location would break this. However, if we need to keep the socket at that location for cases like this I don’t understand how this change would be helpful in any way.

Is it safe to change the default location for this socket and removing the original location?

Thanks in advanced, Devin Lehmacher