git-rev-parse: Allow a "zeroth" parent of a commit - the commit itself.

This sounds nonsensical, but it's useful to make sure that the result is
a commit.

For example, "git-rev-parse v2.6.12" will return the _tag_ object for
v2.6.12, but "git-rev-parse v2.6.12^0" will return the _commit_ object
associated with that tag (and v2.6.12^1 will return the first parent).

Also, since the "parent" code will actually parse the commit, this,
together with the "--verify" flag, will verify not only that the result
is a single SHA1, but will also have verified that it's a proper commit
that we can see.

rev-parse.c

@@ -97,6 +97,10 @@ static int get_parent(char *name, unsigned char *result, int idx)
 		return -1;
 	if (parse_commit(commit))
 		return -1;
+	if (!idx) {
+		memcpy(result, commit->object.sha1, 20);
+		return 0;
+	}
 	p = commit->parents;
 	while (p) {
 		if (!--idx) {
@@ -238,7 +242,7 @@ static int get_extended_sha1(char *name, unsigned char *sha1)
 	int len = strlen(name);
 
 	parent = 1;
-	if (len > 2 && name[len-1] >= '1' && name[len-1] <= '9') {
+	if (len > 2 && name[len-1] >= '0' && name[len-1] <= '9') {
 		parent = name[len-1] - '0';
 		len--;
 	}