Add support for negative refs

[Linus Torvalds <torvalds@osdl.org>]

You can remove a ref that is packed two different ways: either simply
repack all the refs without that one, or create a loose ref that has the
magic all-zero SHA1.

This also adds back the test that a ref actually has the object it
points to.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---

This one is trivial, and obviously depends on the previous series.

I'm sending it just because I have a much bigger cleanup that I'd _really_ 
like to go in, and I did this trivial one before that much more 
interesting one.

diff --git a/refs.c b/refs.c
index 5f80a68..72e2283 100644
--- a/refs.c
+++ b/refs.c
@@ -280,6 +280,12 @@ static int do_for_each_ref(const char *b
 		}
 		if (strncmp(base, entry->name, trim))
 			continue;
+		if (is_null_sha1(entry->sha1))
+			continue;
+		if (!has_sha1_file(entry->sha1)) {
+			error("%s does not point to a valid object!", entry->name);
+			continue;
+		}
 		retval = fn(entry->name + trim, entry->sha1);
 		if (retval)
 			return retval;