Do check_repository_format() early (re-fix)

Do check_repository_format() early (re-fix)

This pushes check_repository_format() (actually _gently() version)
to setup_git_directory_gently() in order to prevent from
using unsupported repositories.

New setup_git_directory_gently()'s behaviour is stop searching
for a valid gitdir and return as if there is no gitdir if a
unsupported repository is found. Warning will be thrown in these
cases.

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