git --bare cmd: do not unconditionally nuke GIT_DIR

git --bare cmd: do not unconditionally nuke GIT_DIR

"GIT_DIR=some.where git --bare cmd" and worse yet
"git --git-dir=some.where --bare cmd" were very confusing.  They
both ignored git-dir specified, and instead made $cwd as GIT_DIR.

This changes --bare not to override existing GIT_DIR.

This has been like this for a long time.  Let's hope nobody sane
relied on this insane behaviour.

Signed-off-by: Junio C Hamano <gitster@pobox.com>