fetch/clone: mark messages from remote side stand out.
When dealing with a corrupt or out of sync remote repository, the user often gets error messages like this:
error: refs/heads/devel does not point to a valid commit object!
which leaves the user wondering if the breakage is on the local end or on the remote end. This is unnecessarily alarming.
This patch changes the way we display messages received from the remote side over the git protocol sideband (i.e. stderr stream of the remote process). It shows them with blue background with white letters, but this presentation is subject to proposals of better ways from the list.