reset: make tree counting less confusing

Depending on whether we're in --keep mode, git-reset may
feed one or two trees to unpack_trees(). We start a counter
at "1" and then increment it to "2" only for the two-tree
case. But that means we must always subtract one to find the
correct array slot to fill with each descriptor.

Instead, let's start at "0" and just increment our counter
after adding each tree. This skips the extra subtraction,
and will make things much easier when we start to actually
free our tree buffers.

While we're at it, let's make the first allocation use the
slot at "desc + nr", too, even though we know "nr" is 0 at
that point. It makes the two fill_tree_descriptor() calls
consistent (always "desc + nr", followed by always
incrementing "nr").

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -44,7 +44,7 @@ static inline int is_merge(void)
 
 static int reset_index(const struct object_id *oid, int reset_type, int quiet)
 {
-	int nr = 1;
+	int nr = 0;
 	struct tree_desc desc[2];
 	struct tree *tree;
 	struct unpack_trees_options opts;
@@ -75,14 +75,16 @@ static int reset_index(const struct object_id *oid, int reset_type, int quiet)
 		struct object_id head_oid;
 		if (get_oid("HEAD", &head_oid))
 			return error(_("You do not have a valid HEAD."));
-		if (!fill_tree_descriptor(desc, &head_oid))
+		if (!fill_tree_descriptor(desc + nr, &head_oid))
 			return error(_("Failed to find tree of HEAD."));
 		nr++;
 		opts.fn = twoway_merge;
 	}
 
-	if (!fill_tree_descriptor(desc + nr - 1, oid))
+	if (!fill_tree_descriptor(desc + nr, oid))
 		return error(_("Failed to find tree of %s."), oid_to_hex(oid));
+	nr++;
+
 	if (unpack_trees(nr, desc, &opts))
 		return -1;