verify_lock(): report errors via a strbuf

Instead of writing error messages directly to stderr, write them to
a "strbuf *err".  The caller, lock_ref_sha1_basic(), uses this error
reporting convention with all the other callees, and reports its
error this way to its callers.

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

refs.c

@@ -2221,23 +2221,28 @@ static void unlock_ref(struct ref_lock *lock)
 /*
  * Verify that the reference locked by lock has the value old_sha1.
  * Fail if the reference doesn't exist and mustexist is set. Return 0
- * on success or a negative value on error. This function should make
- * sure errno is meaningful on error.
+ * on success. On error, write an error message to err, set errno, and
+ * return a negative value.
  */
 static int verify_lock(struct ref_lock *lock,
-		       const unsigned char *old_sha1, int mustexist)
+		       const unsigned char *old_sha1, int mustexist,
+		       struct strbuf *err)
 {
+	assert(err);
+
 	if (read_ref_full(lock->ref_name,
 			  mustexist ? RESOLVE_REF_READING : 0,
 			  lock->old_sha1, NULL)) {
 		int save_errno = errno;
-		error("Can't verify ref %s", lock->ref_name);
+		strbuf_addf(err, "Can't verify ref %s", lock->ref_name);
 		errno = save_errno;
 		return -1;
 	}
 	if (hashcmp(lock->old_sha1, old_sha1)) {
-		error("Ref %s is at %s but expected %s", lock->ref_name,
-			sha1_to_hex(lock->old_sha1), sha1_to_hex(old_sha1));
+		strbuf_addf(err, "Ref %s is at %s but expected %s",
+			    lock->ref_name,
+			    sha1_to_hex(lock->old_sha1),
+			    sha1_to_hex(old_sha1));
 		errno = EBUSY;
 		return -1;
 	}
@@ -2469,7 +2474,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 			goto error_return;
 		}
 	}
-	if (old_sha1 && verify_lock(lock, old_sha1, mustexist)) {
+	if (old_sha1 && verify_lock(lock, old_sha1, mustexist, err)) {
 		last_errno = errno;
 		goto error_return;
 	}