Merge branch 'mh/reporting-broken-refs-from-for-each-ref'

"git for-each-ref" reported "missing object" for 0{40} when it
encounters a broken ref.  The lack of object whose name is 0{40} is
not the problem; the ref being broken is.

* mh/reporting-broken-refs-from-for-each-ref:
read_loose_refs(): treat NULL_SHA1 loose references as broken
read_loose_refs(): simplify function logic
for-each-ref: report broken references correctly
t6301: new tests of for-each-ref error handling

diff --cc builtin/for-each-ref.c
Simple merge

diff --cc refs.c
index 26d1ac1e32eb4fc4c6729abcf0354dbad6177d1b,07f8847e6d78423a4b70b9e66d6ddfe97845e421..7ac05cf21a25802f8e16d45ef75e37b7de2cda8a
--- 1/refs.c
--- 2/refs.c
+++ b/refs.c
@@@ -1376,20 -1286,31 +1378,33 @@@ static void read_loose_refs(const char 
                        if (*refs->name) {
                                hashclr(sha1);
                                flag = 0;
-                               if (resolve_gitlink_ref(refs->name, refname.buf, sha1) < 0) {
-                                       hashclr(sha1);
-                                       flag |= REF_ISBROKEN;
-                               }
-                       } else if (read_ref_full(refname.buf,
-                                                RESOLVE_REF_READING,
-                                                sha1, &flag)) {
+                               read_ok = !resolve_gitlink_ref(refs->name,
+                                                              refname.buf, sha1);
+                       } else {
+                               read_ok = !read_ref_full(refname.buf,
+                                                        RESOLVE_REF_READING,
+                                                        sha1, &flag);
+                       }
+ 
+                       if (!read_ok) {
                                hashclr(sha1);
                                flag |= REF_ISBROKEN;
+                       } else if (is_null_sha1(sha1)) {
+                               /*
+                                * It is so astronomically unlikely
+                                * that NULL_SHA1 is the SHA-1 of an
+                                * actual object that we consider its
+                                * appearance in a loose reference
+                                * file to be repo corruption
+                                * (probably due to a software bug).
+                                */
+                               flag |= REF_ISBROKEN;
                        }
+ 
                        if (check_refname_format(refname.buf,
                                                 REFNAME_ALLOW_ONELEVEL)) {
 +                              if (!refname_is_safe(refname.buf))
 +                                      die("loose refname is dangerous: %s", refname.buf);
                                hashclr(sha1);
                                flag |= REF_BAD_NAME | REF_ISBROKEN;
                        }