git-sh-setup: move the repository check to a core program.

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>

git-sh-setup.sh

@@ -19,10 +19,5 @@ die() {
 	exit 1
 }
 
-case "$(GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD 2>/dev/null)" in
-refs/*)	: ;;
-*)	false ;;
-esac &&
-[ -d "$GIT_DIR/refs" ] &&
-[ -d "$GIT_OBJECT_DIRECTORY/" ] ||
-	die "Not a git repository."
+# Make sure we are in a valid repository of a vintage we understand.
+GIT_DIR="$GIT_DIR" git-var GIT_AUTHOR_IDENT >/dev/null || exit