Make networking commands to work from a subdirectory.

Make networking commands to work from a subdirectory.

These are whole-tree operations and there is not much point
making them operable from within a subdirectory, but it is easy
to do so, and using setup_git_directory() upfront helps git://
proxy specification picked up from the correct place.

Signed-off-by: Junio C Hamano <junkio@cox.net>