Make iterator ignore eval lazy

This makes it so that the check if a file is ignored will be
deferred until requested on the workdir iterator, instead of
aggressively evaluating the ignore rules for each entry.  This
should improve performance because there will be no need to
check ignore rules for files that are already in the index.

src/iterator.c

@@ -641,13 +641,12 @@ static int workdir_iterator__update_entry(workdir_iterator *wi)
 
 	wi->entry.path = ps->path;
 
-	/* skip over .git entry */
+	/* skip over .git entries */
 	if (STRCMP_CASESELECT(wi->base.ignore_case, ps->path, DOT_GIT "/") == 0 ||
 		STRCMP_CASESELECT(wi->base.ignore_case, ps->path, DOT_GIT) == 0)
 		return workdir_iterator__advance((git_iterator *)wi, NULL);
 
-	/* if there is an error processing the entry, treat as ignored */
-	wi->is_ignored = 1;
+	wi->is_ignored = -1;
 
 	git_index__init_entry_from_stat(&ps->st, &wi->entry);
 
@@ -655,12 +654,10 @@ static int workdir_iterator__update_entry(workdir_iterator *wi)
 	wi->entry.mode = git_futils_canonical_mode(ps->st.st_mode);
 
 	/* if this is a file type we don't handle, treat as ignored */
-	if (wi->entry.mode == 0)
+	if (wi->entry.mode == 0) {
+		wi->is_ignored = 1;
 		return 0;
-
-	/* okay, we are far enough along to look up real ignore rule */
-	if (git_ignore__lookup(&wi->ignores, wi->entry.path, &wi->is_ignored) < 0)
-		return 0; /* if error, ignore it and ignore file */
+	}
 
 	/* detect submodules */
 	if (S_ISDIR(wi->entry.mode)) {
@@ -908,8 +905,18 @@ notfound:
 
 int git_iterator_current_is_ignored(git_iterator *iter)
 {
-	return (iter->type != GIT_ITERATOR_WORKDIR) ? 0 :
-		((workdir_iterator *)iter)->is_ignored;
+	workdir_iterator *wi = (workdir_iterator *)iter;
+
+	if (iter->type != GIT_ITERATOR_WORKDIR)
+		return 0;
+
+	if (wi->is_ignored != -1)
+		return wi->is_ignored;
+
+	if (git_ignore__lookup(&wi->ignores, wi->entry.path, &wi->is_ignored) < 0)
+		wi->is_ignored = 1;
+
+	return wi->is_ignored;
 }
 
 int git_iterator_advance_into_directory(