Add git-symbolic-ref

This adds the counterpart of git-update-ref that lets you read
and create "symbolic refs".  By default it uses a symbolic link
to represent ".git/HEAD -> refs/heads/master", but it can be compiled
to use the textfile symbolic ref.

The places that did 'readlink .git/HEAD' and 'ln -s refs/heads/blah
.git/HEAD' have been converted to use new git-symbolic-ref command, so
that they can deal with either implementation.

Signed-off-by: Junio C Hamano <junio@twinsun.com>

git-sh-setup.sh

@@ -13,10 +13,13 @@
 unset CDPATH
 
 die() {
-	echo "$@" >&2
+	echo >&2 "$@"
 	exit 1
 }
 
-[ -h "$GIT_DIR/HEAD" ] &&
+case "$(GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD 2>/dev/null)" in
+refs/*)	: ;;
+*)	false ;;
+esac &&
 [ -d "$GIT_DIR/refs" ] &&
 [ -d "$GIT_OBJECT_DIRECTORY/00" ]