diff conflicts: don't include incorrect ID

Since a diff entry only concerns a single entry, zero the information
for the index side of a conflict.  (The index entry would otherwise
erroneously include the lowest-stage index entry - generally the
ancestor of a conflict.)

Test that during status, the index side of the conflict is empty.

1 files changed, 21 insertions, 16 deletions

diff --git a/src/diff.c b/src/diff.c
index c061a903b..abe0de0d4 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -149,9 +149,10 @@ static int diff_delta__from_two(
 	uint32_t old_mode,
 	const git_index_entry *new_entry,
 	uint32_t new_mode,
-	git_oid *new_oid,
+	const git_oid *new_id,
 	const char *matched_pathspec)
 {
+	const git_oid *old_id = &old_entry->id;
 	git_diff_delta *delta;
 	const char *canonical_path = old_entry->path;
 
@@ -159,38 +160,42 @@ static int diff_delta__from_two(
 		DIFF_FLAG_ISNT_SET(diff, GIT_DIFF_INCLUDE_UNMODIFIED))
 		return 0;
 
+	if (!new_id)
+		new_id = &new_entry->id;
+
 	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_REVERSE)) {
 		uint32_t temp_mode = old_mode;
 		const git_index_entry *temp_entry = old_entry;
+		const git_oid *temp_id = old_id;
+
 		old_entry = new_entry;
 		new_entry = temp_entry;
 		old_mode = new_mode;
 		new_mode = temp_mode;
+		old_id = new_id;
+		new_id = temp_id;
 	}
 
 	delta = diff_delta__alloc(diff, status, canonical_path);
 	GITERR_CHECK_ALLOC(delta);
 	delta->nfiles = 2;
 
-	git_oid_cpy(&delta->old_file.id, &old_entry->id);
-	delta->old_file.size = old_entry->file_size;
-	delta->old_file.mode = old_mode;
-	delta->old_file.flags |= GIT_DIFF_FLAG_VALID_ID;
+	if (!git_index_entry_stage(old_entry)) {
+		delta->old_file.size = old_entry->file_size;
+		delta->old_file.mode = old_mode;
+		git_oid_cpy(&delta->old_file.id, old_id);
+		delta->old_file.flags |= GIT_DIFF_FLAG_VALID_ID;
+	}
 
-	git_oid_cpy(&delta->new_file.id, &new_entry->id);
-	delta->new_file.size = new_entry->file_size;
-	delta->new_file.mode = new_mode;
+	if (!git_index_entry_stage(new_entry)) {
+		git_oid_cpy(&delta->new_file.id, new_id);
+		delta->new_file.size = new_entry->file_size;
+		delta->new_file.mode = new_mode;
 
-	if (new_oid) {
-		if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_REVERSE))
-			git_oid_cpy(&delta->old_file.id, new_oid);
-		else
-			git_oid_cpy(&delta->new_file.id, new_oid);
+		if (!git_oid_iszero(&new_entry->id))
+			delta->new_file.flags |= GIT_DIFF_FLAG_VALID_ID;
 	}
 
-	if (new_oid || !git_oid_iszero(&new_entry->id))
-		delta->new_file.flags |= GIT_DIFF_FLAG_VALID_ID;
-
 	return diff_insert_delta(diff, delta, matched_pathspec);
 }