Make it possible to set up libgit directly (instead of from the environment)

Make it possible to set up libgit directly (instead of from the environment)

This introduces a setup_git() function which is essentialy a (public)
backend for setup_git_env() which lets anyone specify custom sources
for the various paths instead of environment variables. Since the repositories
may get switched on the fly, this also updates code that caches paths to
invalidate them properly; I hope neither of those is a sweet spot.

It is used by Git.xs' xs__call_gate() to set up per-repository data
for libgit's consumption. No code actually takes advantage of it yet
but get_object() will in the next patches.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>