revised^2: git-daemon extra paranoia, and path DWIM

revised^2: git-daemon extra paranoia, and path DWIM

This patch adds some extra paranoia to the git-daemon filename test.  In
particular, it now rejects pathnames containing //; it also adds a
redundant test for pathname absoluteness (belts and suspenders.)

A single / at the end of the path is still permitted, however, and the
.git and /.git append DWIM stuff is now handled in an integrated manner,
which means the resulting path will always be subjected to pathname checks.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>