git-apply: skip threeway in add / rename cases

Certain invocations of "git apply --3way" will attempt threeway and
fail due to missing objects, even though git is able to fall back on
apply_fragments and apply the patch successfully with a return value
of 0. To fix, return early from try_threeway() in the following
cases:

 - When the patch is a rename and no lines have changed. In this
   case, "git diff" doesn't record the blob info, so 3way is neither
   possible nor necessary.

 - When the patch is an addition and there is no add/add conflict,
   i.e. direct_to_threeway is false. In this case, threeway will
   fail since the preimage is not in cache, but isn't necessary
   anyway since there is no conflict.

This fixes a few unecessary error messages when applying these kinds
of patches with --3way.

It also fixes a reported issue where applying a concatenation of
several git produced patches will fail when those patches involve a
deletion followed by creation of the same file.  Add a test for this
case too.  (test provided by <[email protected]>)

Signed-off-by: Jerry Zhang <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jerry-skydio

apply.c

@@ -3582,7 +3582,9 @@ static int try_threeway(struct apply_state *state,
 
 	/* No point falling back to 3-way merge in these cases */
 	if (patch->is_delete ||
-	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode))
+	    S_ISGITLINK(patch->old_mode) || S_ISGITLINK(patch->new_mode) ||
+	    (patch->is_new && !patch->direct_to_threeway) ||
+	    (patch->is_rename && !patch->lines_added && !patch->lines_deleted))
 		return -1;
 
 	/* Preimage the patch was prepared for */

t/t4108-apply-threeway.sh

@@ -275,4 +275,22 @@ test_expect_success 'apply full-index patch with 3way' '
 	git apply --3way --index bin.diff
 '
 
+test_expect_success 'apply delete then new patch with 3way' '
+	git reset --hard main &&
+	test_write_lines 2 > delnew &&
+	git add delnew &&
+	git diff --cached >> new.patch &&
+	git reset --hard &&
+	test_write_lines 1 > delnew &&
+	git add delnew &&
+	git commit -m "delnew" &&
+	rm delnew &&
+	git diff >> delete-then-new.patch &&
+	cat new.patch >> delete-then-new.patch &&
+
+	git checkout -- . &&
+	# Apply must succeed.
+	git apply --3way delete-then-new.patch
+'
+
 test_done