merge-ort: defer recursing into directories when merge base is matched

When one side of history matches the merge base (including when the
merge base has no entry for the given directory), have
collect_merge_info_callback() defer recursing into the directory.  To
ensure those entries are eventually handled, add a call to
handled_deferred_entries() in collect_merge_info() after
traverse_trees() returns.

Note that the condition in collect_merge_info_callback() may look more
complicated than necessary at first glance;
renames->trivial_merges_okay[side] is always true until
handle_deferred_entries() is called, and possible_trivial_merges[side]
is always empty right now (and in the future won't be filled until
handle_deferred_entries() is called).  However, when
handle_deferred_entries() calls traverse_trees() for the relevant
deferred directories, those traverse_trees() calls will once again end
up in collect_merge_info_callback() for all the entries under those
subdirectories.  The extra conditions are there for such deferred cases
and will be used more as we do more with those variables.

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

merge-ort.c

@@ -1147,8 +1147,36 @@ static int collect_merge_info_callback(int n,
 		struct tree_desc t[3];
 		void *buf[3] = {NULL, NULL, NULL};
 		const char *original_dir_name;
-		int i, ret;
+		int i, ret, side;
 
+		/*
+		 * Check for whether we can avoid recursing due to one side
+		 * matching the merge base.  The side that does NOT match is
+		 * the one that might have a rename destination we need.
+		 */
+		assert(!side1_matches_mbase || !side2_matches_mbase);
+		side = side1_matches_mbase ? MERGE_SIDE2 :
+			side2_matches_mbase ? MERGE_SIDE1 : MERGE_BASE;
+		if (filemask == 0 && (dirmask == 2 || dirmask == 4)) {
+			/*
+			 * Also defer recursing into new directories; set up a
+			 * few variables to let us do so.
+			 */
+			ci->match_mask = (7 - dirmask);
+			side = dirmask / 2;
+		}
+		if (renames->dir_rename_mask != 0x07 &&
+		    side != MERGE_BASE &&
+		    renames->deferred[side].trivial_merges_okay &&
+		    !strset_contains(&renames->deferred[side].target_dirs,
+				     pi.string)) {
+			strintmap_set(&renames->deferred[side].possible_trivial_merges,
+				      pi.string, renames->dir_rename_mask);
+			renames->dir_rename_mask = prev_dir_rename_mask;
+			return mask;
+		}
+
+		/* We need to recurse */
 		ci->match_mask &= filemask;
 		newinfo = *info;
 		newinfo.prev = info;
@@ -1202,7 +1230,6 @@ static int collect_merge_info_callback(int n,
 	return mask;
 }
 
-MAYBE_UNUSED
 static int handle_deferred_entries(struct merge_options *opt,
 				   struct traverse_info *info)
 {
@@ -1291,6 +1318,8 @@ static int collect_merge_info(struct merge_options *opt,
 
 	trace2_region_enter("merge", "traverse_trees", opt->repo);
 	ret = traverse_trees(NULL, 3, t, &info);
+	if (ret == 0)
+		ret = handle_deferred_entries(opt, &info);
 	trace2_region_leave("merge", "traverse_trees", opt->repo);
 
 	return ret;