clone: drop extra newline from warning message

clone: drop extra newline from warning message

We don't need to put a "\n" in calls to warning(), since it adds one
itself (and the user sees an extra blank line). Drop it, and while we're
here, drop the full-stop from the message, which goes against our
guidelines.

This bug dates all the way back to 8434c2f1af (Build in clone,
2008-04-27), but presumably nobody noticed because it's hard to trigger:
you have to clone a repository whose HEAD is unborn, but which is not
otherwise empty.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>