Avoid scary errors about tagged trees/blobs during git-fetch

Avoid scary errors about tagged trees/blobs during git-fetch

Ok, what is going on is:

 - append_fetch_head() looks up the SHA1 for all heads (including tags):

        if (get_sha1(head, sha1))
                return error("Not a valid object name: %s", head);

 - it then wants to check if it's a candidate for merging (because
   fetching also does the whole "list which heads to merge" in case
   it is going to be part of a "pull"):

        commit = lookup_commit_reference(sha1);
        if (!commit)
                not_for_merge = 1;

 - and that "lookup_commit_reference()" is just very vocal about the
   case where it fails. It really shouldn't be, and it shouldn't
   affect the actual end result, but that basically explains why
   you get that scary warning.

In short, the warning is just bogus, and should be harmless, but
I agree that it's ugly. I think the appended patch should fix it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>