Fix some of the glaring errors in GIT_DIFF_REVERSE

These changes fix the basic problem with GIT_DIFF_REVERSE being
broken for text diffs.  The reversed diff entries were getting
added to the git_diff correctly, but some of the metadata was kept
incorrectly in a way that prevented the text diffs from being
generated correctly.  Once I fixed that, it became clear that it
was not possible to merge reversed diffs correctly.  This has a
first pass at fixing that problem.  We probably need more tests
to make sure that is really fixed thoroughly.

src/diff.c

@@ -448,6 +448,13 @@ static int diff_list_apply_options(
 		/* add other defaults here */
 	}
 
+	/* Reverse src info if diff is reversed */
+	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_REVERSE)) {
+		git_iterator_type_t tmp_src = diff->old_src;
+		diff->old_src = diff->new_src;
+		diff->new_src = tmp_src;
+	}
+
 	/* if ignore_submodules not explicitly set, check diff config */
 	if (diff->opts.ignore_submodules <= 0) {
 		const char *str;
@@ -484,9 +491,9 @@ static int diff_list_apply_options(
 		return -1;
 
 	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_REVERSE)) {
-		const char *swap = diff->opts.old_prefix;
+		const char *tmp_prefix = diff->opts.old_prefix;
 		diff->opts.old_prefix  = diff->opts.new_prefix;
-		diff->opts.new_prefix = swap;
+		diff->opts.new_prefix  = tmp_prefix;
 	}
 
 	return 0;

src/diff_tform.c

@@ -46,7 +46,10 @@ fail:
 }
 
 static git_diff_delta *diff_delta__merge_like_cgit(
-	const git_diff_delta *a, const git_diff_delta *b, git_pool *pool)
+	uint16_t flags,
+	const git_diff_delta *a,
+	const git_diff_delta *b,
+	git_pool *pool)
 {
 	git_diff_delta *dup;
 
@@ -112,26 +115,28 @@ int git_diff_merge(
 	if (!from->deltas.length)
 		return 0;
 
+	if ((onto->opts.flags & GIT_DIFF_IGNORE_CASE) !=
+		(from->opts.flags & GIT_DIFF_IGNORE_CASE) ||
+		(onto->opts.flags & GIT_DIFF_REVERSE) !=
+		(from->opts.flags & GIT_DIFF_REVERSE))
+	{
+		giterr_set(GITERR_INVALID,
+			"Attempt to merge diffs created with conflicting options");
+		return -1;
+	}
+
 	if (git_vector_init(
 			&onto_new, onto->deltas.length, git_diff_delta__cmp) < 0 ||
 		git_pool_init(&onto_pool, 1, 0) < 0)
 		return -1;
 
-	if ((onto->opts.flags & GIT_DIFF_IGNORE_CASE) != 0 ||
-		(from->opts.flags & GIT_DIFF_IGNORE_CASE) != 0)
-	{
-		ignore_case = true;
-
-		/* This function currently only supports merging diff lists that
-		 * are sorted identically. */
-		assert((onto->opts.flags & GIT_DIFF_IGNORE_CASE) != 0 &&
-				(from->opts.flags & GIT_DIFF_IGNORE_CASE) != 0);
-	}
+	ignore_case = ((onto->opts.flags & GIT_DIFF_IGNORE_CASE) != 0);
 
 	for (i = 0, j = 0; i < onto->deltas.length || j < from->deltas.length; ) {
 		git_diff_delta *o = GIT_VECTOR_GET(&onto->deltas, i);
 		const git_diff_delta *f = GIT_VECTOR_GET(&from->deltas, j);
-		int cmp = !f ? -1 : !o ? 1 : STRCMP_CASESELECT(ignore_case, o->old_file.path, f->old_file.path);
+		int cmp = !f ? -1 : !o ? 1 :
+			STRCMP_CASESELECT(ignore_case, o->old_file.path, f->old_file.path);
 
 		if (cmp < 0) {
 			delta = diff_delta__dup(o, &onto_pool);
@@ -140,7 +145,8 @@ int git_diff_merge(
 			delta = diff_delta__dup(f, &onto_pool);
 			j++;
 		} else {
-			delta = diff_delta__merge_like_cgit(o, f, &onto_pool);
+			delta = diff_delta__merge_like_cgit(
+				onto->opts.flags, o, f, &onto_pool);
 			i++;
 			j++;
 		}
@@ -160,6 +166,10 @@ int git_diff_merge(
 	if (!error) {
 		git_vector_swap(&onto->deltas, &onto_new);
 		git_pool_swap(&onto->pool, &onto_pool);
+
+		if ((onto->opts.flags & GIT_DIFF_REVERSE) != 0)
+			onto->old_src = from->old_src;
+		else
 			onto->new_src = from->new_src;
 
 		/* prefix strings also come from old pool, so recreate those.*/