rerere forget: deprecate invocation without pathspec

rerere forget: deprecate invocation without pathspec

rerere forget is a destructive command. When invoked without a path, it
operates on the current directory, potentially deleting many recorded
conflict resolutions.

To make the command safer, a path must be specified as of git 1.8.0. Until
then, give users time to write 'git rerere forget .' if they really mean
the entire current directory.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>