git.txt: add missing info about --git-dir command-line option
git.txt: add missing info about --git-dir command-line option

Unlike other environment variables (e.g. GIT_WORK_TREE,	GIT_NAMESPACE),
the Documentation/git.txt file did not mention that the GIT_DIR
environment variable can also be set using the --git-dir command line
option.

Signed-off-by: Manlio Perillo <6c2efbaf77efbcd74c709763b78b48f750ed9700@gmail.com>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git