enter_repo: allow .git files in strict mode

enter_repo: allow .git files in strict mode

Strict mode is about not guessing where .git is. If the user points to a
.git file, we know exactly where the target .git dir will be. This makes
it possible to serve .git files as repository on the server side.

This may be needed even in local clone case because transport.c code
uses upload-pack for fetching remote refs. But right now the
clone/transport code goes with non-strict.

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