Tests and more fixes for submodule diffs

This adds tests for diffs with submodules in them and (perhaps
unsurprisingly) requires further fixes to be made.  Specifically,
this fixes:

- when considering if a submodule is dirty in the workdir, it was
  being treated as dirty even if only the index was dirty.
- git_diff_patch_to_str (and git_diff_patch_print) were "printing"
  the headers for files (and submodules) that were unmodified or
  had no meaningful content.
- added comment to previous fix and removed unneeded parens.

1 files changed, 10 insertions, 3 deletions

diff --git a/src/diff_output.c b/src/diff_output.c
index d462142f9..34a3e506c 100644
--- a/src/diff_output.c
+++ b/src/diff_output.c
@@ -675,11 +675,14 @@ cleanup:
 	if (!error) {
 		patch->flags |= GIT_DIFF_PATCH_LOADED;
 
+		/* patch is diffable only for non-binary, modified files where at
+		 * least one side has data and there is actual change in the data
+		 */
 		if ((delta->flags & GIT_DIFF_FLAG_BINARY) == 0 &&
 			delta->status != GIT_DELTA_UNMODIFIED &&
 			(patch->old_data.len || patch->new_data.len) &&
-			((patch->old_data.len != patch->new_data.len) ||
-				!git_oid_equal(&delta->old_file.oid, &delta->new_file.oid)))
+			(patch->old_data.len != patch->new_data.len ||
+			 !git_oid_equal(&delta->old_file.oid, &delta->new_file.oid)))
 			patch->flags |= GIT_DIFF_PATCH_DIFFABLE;
 	}
 
@@ -1150,7 +1153,11 @@ static int print_patch_file(
 
 	GIT_UNUSED(progress);
 
-	if (S_ISDIR(delta->new_file.mode))
+	if (S_ISDIR(delta->new_file.mode) ||
+		delta->status == GIT_DELTA_UNMODIFIED ||
+		delta->status == GIT_DELTA_IGNORED ||
+		(delta->status == GIT_DELTA_UNTRACKED &&
+		 (pi->diff->opts.flags & GIT_DIFF_INCLUDE_UNTRACKED_CONTENT) == 0))
 		return 0;
 
 	if (!oldpfx)