Shallow clone: do not ignore shallowness when following tags

Tags should be considered when truncating the
commit list. The patch below fixes it, and fetches the right number of
commits for each tag. However the correct fix is probably to not fetch
historical tags at all.

Signed-off-by: Junio C Hamano <junkio@cox.net>

shallow.c

@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "commit.h"
+#include "tag.h"
 
 static int is_shallow = -1;
 
@@ -54,7 +55,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
 		if (!commit) {
 			if (i < heads->nr) {
 				commit = (struct commit *)
-					heads->objects[i++].item;
+					deref_tag(heads->objects[i++].item, NULL, 0);
 				if (commit->object.type != OBJ_COMMIT) {
 					commit = NULL;
 					continue;