Bug: bad errno report from lock_ref_sha1_basic() when contents bad

Hi Junio,

User "mbrochh" on IRC reported a result along the lines of

  error: unable to resolve reference refs/remotes/origin/master: Success
  ! [new branch] master -> origin/master (unable to update local ref)

from 'git fetch' (or pull actually, but clearly it's the fetch part failing). This error message is from lock_ref_sha1_basic(), and suggests a confused error exit taken with errno=0.

I can reproduce the problem by writing a bit of garbage to a remote ref and then fetching, though I usually get some other error in the message (usually ENOENT, which is equally wrong).

As far as I can tell it's caused by resolve_ref_unsafe() returning NULL when the content checks fail:

	if (get_sha1_hex(buffer, sha1) || (buffer[40] != '\0' && !isspace(buffer[40]))) {
		if (flag)
			*flag |= REF_ISBROKEN;
		return NULL;
	}

This used to warn before your 5595635 (resolve_ref(): report breakage to the caller without warning, 2011-10-19), but otherwise suffered from the same problem.

As far as I can see lock_ref_sha1_basic() needs a similar check for REF_ISBROKEN (and presumably the only fix is to nuke it so maybe it should even suggest that to the user). But in general, the "errno may be crucial or worthless" interface of lock_ref_sha1_basic() seems a bit broken...