merge-ort: handle interactions of caching and rename/rename(1to1) cases

As documented in Documentation/technical/remembering-renames.txt, and as
tested for in the two testcases in t6429 with "rename same file
identically" in their description, there is one case where we need to
have renames in one commit NOT be cached for the next commit in our
rebase sequence -- namely, rename/rename(1to1) cases.  Rather than
specifically trying to uncache those and fix up dir_rename_counts() to
match (which would also be valid but more work), we simply disable the
optimization when this really rare type of rename occurs.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort.c

@@ -2111,6 +2111,9 @@ static int process_renames(struct merge_options *opt,
 			VERIFY_CI(side2);
 
 			if (!strcmp(pathnames[1], pathnames[2])) {
+				struct rename_info *ri = &opt->priv->renames;
+				int j;
+
 				/* Both sides renamed the same way */
 				assert(side1 == side2);
 				memcpy(&side1->stages[0], &base->stages[0],
@@ -2120,6 +2123,16 @@ static int process_renames(struct merge_options *opt,
 				base->merged.is_null = 1;
 				base->merged.clean = 1;
 
+				/*
+				 * Disable remembering renames optimization;
+				 * rename/rename(1to1) is incredibly rare, and
+				 * just disabling the optimization is easier
+				 * than purging cached_pairs,
+				 * cached_target_names, and dir_rename_counts.
+				 */
+				for (j = 0; j < 3; j++)
+					ri->merge_trees[j] = NULL;
+
 				/* We handled both renames, i.e. i+1 handled */
 				i++;
 				/* Move to next rename */
@@ -3918,7 +3931,22 @@ static void merge_check_renames_reusable(struct merge_options *opt,
 
 	renames = &opti->renames;
 	merge_trees = renames->merge_trees;
-	/* merge_trees[0..2] will only be NULL if opti is */
+
+	/*
+	 * Handle case where previous merge operation did not want cache to
+	 * take effect, e.g. because rename/rename(1to1) makes it invalid.
+	 */
+	if (!merge_trees[0]) {
+		assert(!merge_trees[0] && !merge_trees[1] && !merge_trees[2]);
+		renames->cached_pairs_valid_side = 0; /* neither side valid */
+		return;
+	}
+
+	/*
+	 * Handle other cases; note that merge_trees[0..2] will only
+	 * be NULL if opti is, or if all three were manually set to
+	 * NULL by e.g. rename/rename(1to1) handling.
+	 */
 	assert(merge_trees[0] && merge_trees[1] && merge_trees[2]);
 
 	/* Check if we meet a condition for re-using cached_pairs */