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-checkout.sh

@@ -71,7 +71,8 @@ if [ "$?" -eq 0 ]; then
 		echo $new > "$GIT_DIR/refs/heads/$newbranch"
 		branch="$newbranch"
 	fi
-	[ "$branch" ] && ln -sf "refs/heads/$branch" "$GIT_DIR/HEAD"
+	[ "$branch" ] &&
+	GIT_DIR="$GIT_DIR" git-symbolic-ref HEAD "refs/heads/$branch"
 	rm -f "$GIT_DIR/MERGE_HEAD"
 else
 	exit 1