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 <i@zenithal.me>)

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/apply.c b/apply.c
index 43a0aebf4e..64b226acd9 100644
--- a/apply.c
+++ b/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 */