merge: disable `m2-vs-ma` optimization if new filenode config is true

The `m2-vs-ma` optimization filters out the file which have not changed between
second parent and the ancestor of the merge. This results in the m1-vs-m2 diff
not processing those files. These files will be required when we are creating
new filenode for salvaged cases as we need to track them to store in mergestate
that file can be salvaged.

Differential Revision: https://phab.mercurial-scm.org/D9027

mercurial/merge.py

@@ -776,7 +776,13 b' def manifestmerge('
     # - ma is the same as m1 or m2, which we're just going to diff again later
     # - The caller specifically asks for a full diff, which is useful during bid
     #   merge.
-    if pa not in ([wctx, p2] + wctx.parents()) and not forcefulldiff:
+    # - we are tracking salvaged files specifically hence should process all
+    #   files
+    if (
+        pa not in ([wctx, p2] + wctx.parents())
+        and not forcefulldiff
+        and not repo.ui.configbool(b'experimental', b'merge-track-salvaged')
+    ):
         # Identify which files are relevant to the merge, so we can limit the
         # total m1-vs-m2 diff to just those files. This has significant
         # performance benefits in large repositories.