Only use GIT_CONFIG in "git config", not other programs

Only use GIT_CONFIG in "git config", not other programs

For everything other than using "git config" to read or write a
git-style config file that isn't the current repo's config file,
GIT_CONFIG was actively detrimental. Rather than argue over which
programs are important enough to have work anyway, just fix all of
them at the root.

Also removes GIT_LOCAL_CONFIG, which would only be useful for programs
that do want to use global git-specific config, but not the repo's own
git-specific config, and want to use some other, presumably
git-specific config. Despite being documented, I can't find any sign that
it was ever used.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>