merge-ort: add implementation of type-changed rename handling

Implement cases where renames are involved in type changes (i.e. the
side of history that didn't rename the file changed its type from a
regular file to a symlink or submodule).  There was some code to handle
this in merge-recursive but only in the special case when the renamed
file had no content changes.  The code here works differently -- it
knows process_entry() can handle mode conflicts, so it does a few
minimal tweaks to ensure process_entry() can just finish the job as
needed.

Signed-off-by: Elijah Newren <newren@gmail.com>

merge-ort.c

@@ -778,7 +778,32 @@ static int process_renames(struct merge_options *opt,
 			 S_ISREG(newinfo->stages[target_index].mode));
 		if (type_changed && collision) {
 			/* special handling so later blocks can handle this */
-			die("Not yet implemented");
+			/*
+			 * if type_changed && collision are both true, then this
+			 * was really a double rename, but one side wasn't
+			 * detected due to lack of break detection.  I.e.
+			 * something like
+			 *    orig: has normal file 'foo'
+			 *    side1: renames 'foo' to 'bar', adds 'foo' symlink
+			 *    side2: renames 'foo' to 'bar'
+			 * In this case, the foo->bar rename on side1 won't be
+			 * detected because the new symlink named 'foo' is
+			 * there and we don't do break detection.  But we detect
+			 * this here because we don't want to merge the content
+			 * of the foo symlink with the foo->bar file, so we
+			 * have some logic to handle this special case.  The
+			 * easiest way to do that is make 'bar' on side1 not
+			 * be considered a colliding file but the other part
+			 * of a normal rename.  If the file is very different,
+			 * well we're going to get content merge conflicts
+			 * anyway so it doesn't hurt.  And if the colliding
+			 * file also has a different type, that'll be handled
+			 * by the content merge logic in process_entry() too.
+			 *
+			 * See also t6430, 'rename vs. rename/symlink'
+			 */
+			collision = 0;
+		}
 		if (source_deleted) {
 			if (target_index == 1) {
 				rename_branch = opt->branch1;
@@ -858,7 +883,11 @@ static int process_renames(struct merge_options *opt,
 			newinfo->pathnames[0] = oldpath;
 			if (type_changed) {
 				/* rename vs. typechange */
-				die("Not yet implemented");
+				/* Mark the original as resolved by removal */
+				memcpy(&oldinfo->stages[0].oid, &null_oid,
+				       sizeof(oldinfo->stages[0].oid));
+				oldinfo->stages[0].mode = 0;
+				oldinfo->filemask &= 0x06;
 			} else if (source_deleted) {
 				/* rename/delete */
 				newinfo->path_conflict = 1;