Make setup_git_env() resolve .git file when $GIT_DIR is not specified

Make setup_git_env() resolve .git file when $GIT_DIR is not specified

This makes reinitializing on a .git file repository work.

This is probably the only case that setup_git_env() (via
set_git_dir()) is called on a .git file. Other cases in
setup_git_dir_gently() and enter_repo() both cover .git file case
explicitly because they need to verify the target repo is valid.

Reported-by: Ximin Luo <infinity0@gmx.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>