ediff: Merges with ancestor: Fix computation of hunks and proposed merge

Hunks were not computed correctly because the diff3 command was
invoked with arguments in an incorrect order.  The correct order is
the local file first, the base (or "ancestor") second and the other
file third.  This erroneous behavior had two consequences.  First, the
output of diff3 would change, since it tries to merge chunks according
to maximal matches between the second and first files, and the second
and third files.  Second, ediff, more precisely, `ediff-do-merge',
would consequently try to merge the reverse of the changes from the
base to the other file.

* lisp/vc/ediff-diff.el (ediff-setup-diff-regions3): In the arguments
to `ediff-exec-process', swap the other file with the ancestor (only
when merging with an ancestor).

(ediff-extract-diffs3): Match the hunk data for the ancestor and the
other file correctly.  The local variable `three-way-comp' indicates
this is a merge with ancestors when it is nil.

(Bug#59182)

1 files changed, 21 insertions, 14 deletions

diff --git a/lisp/vc/ediff-diff.el b/lisp/vc/ediff-diff.el
index 07b853817d1..24647de576d 100644
--- a/lisp/vc/ediff-diff.el
+++ b/lisp/vc/ediff-diff.el
@@ -942,6 +942,9 @@ one optional arguments, diff-number to refine.")
 	(c-prev-pt nil)
 	(anc-prev 1)
 	diff-list shift-A shift-B shift-C
+        (A-idx "1")
+        (B-idx (if three-way-comp "2" "3"))
+        (C-idx (if three-way-comp "3" "2"))
 	)
 
     ;; diff list contains word numbers or points, depending on word-mode
@@ -979,23 +982,23 @@ one optional arguments, diff-number to refine.")
        (let ((agreement (buffer-substring (match-beginning 1) (match-end 1))))
 	 ;; if the files A and B are the same and not 3way-comparison,
 	 ;; ignore the difference
-	 (if (or three-way-comp (not (string-equal agreement "3")))
-	     (let* ((a-begin (car (ediff-get-diff3-group "1")))
-		    (a-end  (nth 1 (ediff-get-diff3-group "1")))
-		    (b-begin (car (ediff-get-diff3-group "2")))
-		    (b-end (nth 1 (ediff-get-diff3-group "2")))
-		    (c-or-anc-begin (car (ediff-get-diff3-group "3")))
-		    (c-or-anc-end (nth 1 (ediff-get-diff3-group "3")))
+	 (if (or three-way-comp (not (string-equal agreement C-idx)))
+	     (let* ((a-begin (car (ediff-get-diff3-group A-idx)))
+		    (a-end  (nth 1 (ediff-get-diff3-group A-idx)))
+		    (b-begin (car (ediff-get-diff3-group B-idx)))
+		    (b-end (nth 1 (ediff-get-diff3-group B-idx)))
+		    (c-or-anc-begin (car (ediff-get-diff3-group C-idx)))
+		    (c-or-anc-end (nth 1 (ediff-get-diff3-group C-idx)))
 		    (state-of-merge
-		     (cond ((string-equal agreement "1") 'prefer-A)
-			   ((string-equal agreement "2") 'prefer-B)
+		     (cond ((string-equal agreement A-idx) 'prefer-A)
+			   ((string-equal agreement B-idx) 'prefer-B)
 			   (t ediff-default-variant)))
 		    (state-of-diff-merge
 		     (if (memq state-of-merge '(default-A prefer-A)) 'B 'A))
 		    (state-of-diff-comparison
-		     (cond ((string-equal agreement "1") 'A)
-			   ((string-equal agreement "2") 'B)
-			   ((string-equal agreement "3") 'C)))
+		     (cond ((string-equal agreement A-idx) 'A)
+			   ((string-equal agreement B-idx) 'B)
+			   ((string-equal agreement C-idx) 'C)))
 		    state-of-ancestor
 		    c-begin c-end
 		    a-begin-pt a-end-pt
@@ -1108,8 +1111,12 @@ one optional arguments, diff-number to refine.")
 	    (get-buffer-create (ediff-unique-buffer-name "*ediff-diff" "*"))))
 
   (message "Computing differences ...")
-  (ediff-exec-process ediff-diff3-program ediff-diff-buffer 'synchronize
-		      ediff-actual-diff3-options file-A file-B file-C)
+  (apply #'ediff-exec-process ediff-diff3-program ediff-diff-buffer 'synchronize
+	 ediff-actual-diff3-options
+         (cons file-A (if ediff-merge-with-ancestor-job
+                          ;; Ancestor must be the middle file
+                          (list file-C file-B)
+                          (list file-B file-C))))
 
   (ediff-prepare-error-list ediff-diff3-ok-lines-regexp ediff-diff-buffer)
   ;;(message "Computing differences ... done")