Improve config file escape sanity checking

I had meant to disallow unknown escape characters in the config file
parser, but instead an unknown escaped character would silently pass
through as itself. That's correct for some cases (notably '\' itself), but
wasn't correct in general.

This fixes it, and makes the parser write a nice error message if the
config file contains bogus escaped characters.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>