peel_ref(): fix return value for non-peelable, not-current reference

The old version was inconsistent: when a reference was
REF_KNOWS_PEELED but with a null peeled value, it returned non-zero
for the current reference but zero for other references.  Change the
behavior for non-current references to match that of current_ref,
which is what callers expect.  Document the behavior.

Current callers only call peel_ref() from within a for_each_ref-style
iteration and only for the current ref; therefore, the buggy code path
was never reached.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -119,7 +119,8 @@ struct ref_value {
 	/*
 	 * If REF_KNOWS_PEELED, then this field holds the peeled value
 	 * of this reference, or null if the reference is known not to
-	 * be peelable.
+	 * be peelable.  See the documentation for peel_ref() for an
+	 * exact definition of "peelable".
 	 */
 	unsigned char peeled[20];
 };
@@ -1340,6 +1341,8 @@ int peel_ref(const char *refname, unsigned char *sha1)
 		struct ref_entry *r = get_packed_ref(refname);
 
 		if (r && (r->flag & REF_KNOWS_PEELED)) {
+			if (is_null_sha1(r->u.value.peeled))
+			    return -1;
 			hashcpy(sha1, r->u.value.peeled);
 			return 0;
 		}

refs.h

@@ -74,6 +74,14 @@ extern void add_packed_ref(const char *refname, const unsigned char *sha1);
 
 extern int ref_exists(const char *);
 
+/*
+ * If refname is a non-symbolic reference that refers to a tag object,
+ * and the tag can be (recursively) dereferenced to a non-tag object,
+ * store the SHA1 of the referred-to object to sha1 and return 0.  If
+ * any of these conditions are not met, return a non-zero value.
+ * Symbolic references are considered unpeelable, even if they
+ * ultimately resolve to a peelable tag.
+ */
 extern int peel_ref(const char *refname, unsigned char *sha1);
 
 /** Locks a "refs/" ref returning the lock on success and NULL on failure. **/