branch: don't fail listing branches if one of the commits wasn't found

When listing branches with ref lookups, if one of the known raw refs
doesn't point to a commit then "git branch" would return error(),
terminating the whole for_each_rawref() iteration and possibly hiding
any remaining refs.

Signed-off-by: Simo Melenius <simo.melenius@iki.fi>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -301,7 +301,7 @@ static int append_ref(const char *refname, const unsigned char *sha1, int flags,
 		commit = lookup_commit_reference_gently(sha1, 1);
 		if (!commit) {
 			cb->ret = error("branch '%s' does not point at a commit", refname);
-			return cb->ret;
+			return 0;
 		}
 
 		/* Filter with with_commit if specified */
@@ -539,6 +539,9 @@ static int print_ref_list(int kinds, int detached, int verbose, int abbrev, stru
 
 	free_ref_list(&ref_list);
 
+	if (cb.ret)
+		error("some refs could not be read");
+
 	return cb.ret;
 }