rev-parse: fix meaning of rev~ vs rev~0.

I think it would make more sense for rev~ to have the same guarantees that
rev^ has, namely to always return a commit. I would also suggest that not
giving a number would have the same effect of defaulting to 1, not 0.

Right now it's a bit illogical, but at least it's an _undocumented_
illogical behaviour.

This patch makes '^' and '~' act the same for the default count (i.e. both
default to 1), and also have the same behaviour for a count of zero.

Before (no discernible pattern):

	[torvalds@woody git]$ git rev-parse v1.5.1 v1.5.1^0 v1.5.1~0 v1.5.1^ v1.5.1~
	45354a57ee
	89815cab95
	45354a57ee
	045f5759c9
	45354a57ee

After (fairly logical):

	[torvalds@woody git]$ git rev-parse v1.5.1 v1.5.1^0 v1.5.1~0 v1.5.1^ v1.5.1~
	45354a57ee
	89815cab95
	89815cab95
	045f5759c9
	045f5759c9

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_name.c

@@ -408,18 +408,22 @@ static int get_nth_ancestor(const char *name, int len,
 			    unsigned char *result, int generation)
 {
 	unsigned char sha1[20];
-	int ret = get_sha1_1(name, len, sha1);
+	struct commit *commit;
+	int ret;
+
+	ret = get_sha1_1(name, len, sha1);
 	if (ret)
 		return ret;
+	commit = lookup_commit_reference(sha1);
+	if (!commit)
+		return -1;
 
 	while (generation--) {
-		struct commit *commit = lookup_commit_reference(sha1);
-
-		if (!commit || parse_commit(commit) || !commit->parents)
+		if (parse_commit(commit) || !commit->parents)
 			return -1;
-		hashcpy(sha1, commit->parents->item->object.sha1);
+		commit = commit->parents->item;
 	}
-	hashcpy(result, sha1);
+	hashcpy(result, commit->object.sha1);
 	return 0;
 }
 
@@ -526,9 +530,8 @@ static int get_sha1_1(const char *name, int len, unsigned char *sha1)
 	int ret, has_suffix;
 	const char *cp;
 
-	/* "name~3" is "name^^^",
-	 * "name~" and "name~0" are name -- not "name^0"!
-	 * "name^" is not "name^0"; it is "name^1".
+	/*
+	 * "name~3" is "name^^^", "name~" is "name~1", and "name^" is "name^1".
 	 */
 	has_suffix = 0;
 	for (cp = name + len - 1; name <= cp; cp--) {
@@ -546,11 +549,10 @@ static int get_sha1_1(const char *name, int len, unsigned char *sha1)
 		cp++;
 		while (cp < name + len)
 			num = num * 10 + *cp++ - '0';
-		if (has_suffix == '^') {
-			if (!num && len1 == len - 1)
-				num = 1;
+		if (!num && len1 == len - 1)
+			num = 1;
+		if (has_suffix == '^')
 			return get_parent(name, len1, sha1, num);
-		}
 		/* else if (has_suffix == '~') -- goes without saying */
 		return get_nth_ancestor(name, len1, sha1, num);
 	}