merge-ort: implement check_for_directory_rename()

This is copied from merge-recursive.c, with minor tweaks due to using strmap
API and the fact that it can use opt->priv->paths to get all pathnames that
exist instead of taking a tree object.

This depends on a new function, handle_path_level_conflicts(), which
just has a placeholder die-not-yet-implemented implementation for now; a
subsequent patch will implement it.

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

merge-ort.c

@@ -864,6 +864,21 @@ static void get_renamed_dir_portion(const char *old_path, const char *new_path,
 	*new_dir = xstrndup(new_path, end_of_new - new_path);
 }
 
+/*
+ * See if there is a directory rename for path, and if there are any file
+ * level conflicts on the given side for the renamed location.  If there is
+ * a rename and there are no conflicts, return the new name.  Otherwise,
+ * return NULL.
+ */
+static char *handle_path_level_conflicts(struct merge_options *opt,
+					 const char *path,
+					 unsigned side_index,
+					 struct strmap_entry *rename_info,
+					 struct strmap *collisions)
+{
+	die("Not yet implemented");
+}
+
 static void increment_count(struct strmap *dir_rename_count,
 			    char *old_dir,
 			    char *new_dir)
@@ -1078,7 +1093,57 @@ static char *check_for_directory_rename(struct merge_options *opt,
 					struct strmap *collisions,
 					int *clean_merge)
 {
-	die("Not yet implemented.");
+	char *new_path = NULL;
+	struct strmap_entry *rename_info;
+	struct strmap_entry *otherinfo = NULL;
+	const char *new_dir;
+
+	if (strmap_empty(dir_renames))
+		return new_path;
+	rename_info = check_dir_renamed(path, dir_renames);
+	if (!rename_info)
+		return new_path;
+	/* old_dir = rename_info->key; */
+	new_dir = rename_info->value;
+
+	/*
+	 * This next part is a little weird.  We do not want to do an
+	 * implicit rename into a directory we renamed on our side, because
+	 * that will result in a spurious rename/rename(1to2) conflict.  An
+	 * example:
+	 *   Base commit: dumbdir/afile, otherdir/bfile
+	 *   Side 1:      smrtdir/afile, otherdir/bfile
+	 *   Side 2:      dumbdir/afile, dumbdir/bfile
+	 * Here, while working on Side 1, we could notice that otherdir was
+	 * renamed/merged to dumbdir, and change the diff_filepair for
+	 * otherdir/bfile into a rename into dumbdir/bfile.  However, Side
+	 * 2 will notice the rename from dumbdir to smrtdir, and do the
+	 * transitive rename to move it from dumbdir/bfile to
+	 * smrtdir/bfile.  That gives us bfile in dumbdir vs being in
+	 * smrtdir, a rename/rename(1to2) conflict.  We really just want
+	 * the file to end up in smrtdir.  And the way to achieve that is
+	 * to not let Side1 do the rename to dumbdir, since we know that is
+	 * the source of one of our directory renames.
+	 *
+	 * That's why otherinfo and dir_rename_exclusions is here.
+	 *
+	 * As it turns out, this also prevents N-way transient rename
+	 * confusion; See testcases 9c and 9d of t6043.
+	 */
+	otherinfo = strmap_get_entry(dir_rename_exclusions, new_dir);
+	if (otherinfo) {
+		path_msg(opt, rename_info->key, 1,
+			 _("WARNING: Avoiding applying %s -> %s rename "
+			   "to %s, because %s itself was renamed."),
+			 rename_info->key, new_dir, path, new_dir);
+		return NULL;
+	}
+
+	new_path = handle_path_level_conflicts(opt, path, side_index,
+					       rename_info, collisions);
+	*clean_merge &= (new_path != NULL);
+
+	return new_path;
 }
 
 static void apply_directory_rename_modifications(struct merge_options *opt,