Any core commands that use setup_git_directory() now check if
given GIT_DIR is really a valid repository, so the same check in
git-sh-setup can use it without reimplementing it in shell.
This commit changes git-sh-setup to use git-var command for
that, although any other commands would do.

Note that we export GIT_DIR explicitly when calling git-var;
without it, the caller of this script would use GIT_DIR that we
return (which is to assume ./.git unless the caller has it
elsewhere) while git-var would go up to find a .git directory in
our parent directories, which would be checking a different
directory from what our callers will be using.
Signed-off-by: NJunio C Hamano <junkio@cox.net>