amend: fix amend with copies in extras stable

amend: fix amend with copies in extras

If copy information is stored only in the commit extras and not in
filelogs, then they get lost on amend if the file wasn't also modified
in the working copy. That's because we create `filectx` object from
the old commit in those cases, and the `.copysource()` of such objects
read only from the filelog. This patch fixes it by always creating a
new `memfilectx` in these cases, passing the calculated copy
information to it.

Differential Revision: https://phab.mercurial-scm.org/D12387

--- a/mercurial/cmdutil.py	Fri Mar 18 21:37:22 2022 -0700
+++ b/mercurial/cmdutil.py	Fri Mar 18 21:15:54 2022 -0700
@@ -2906,7 +2906,14 @@
         filestoamend = {f for f in wctx.files() if matcher(f)}
 
         changes = len(filestoamend) > 0
-        if changes:
+        changeset_copies = (
+            repo.ui.config(b'experimental', b'copies.read-from')
+            != b'filelog-only'
+        )
+        # If there are changes to amend or if copy information needs to be read
+        # from the changeset extras, we cannot take the fast path of using
+        # filectxs from the old commit.
+        if changes or changeset_copies:
             # Recompute copies (avoid recording a -> b -> a)
             copied = copies.pathcopies(base, wctx, matcher)
             if old.p2:
@@ -2927,19 +2934,19 @@
 
             def filectxfn(repo, ctx_, path):
                 try:
+                    # Return None for removed files.
+                    if path in wctx.removed():
+                        return None
+
                     # If the file being considered is not amongst the files
-                    # to be amended, we should return the file context from the
+                    # to be amended, we should use the file context from the
                     # old changeset. This avoids issues when only some files in
                     # the working copy are being amended but there are also
                     # changes to other files from the old changeset.
-                    if path not in filestoamend:
-                        return old.filectx(path)
-
-                    # Return None for removed files.
-                    if path in wctx.removed():
-                        return None
-
-                    fctx = wctx[path]
+                    if path in filestoamend:
+                        fctx = wctx[path]
+                    else:
+                        fctx = old.filectx(path)
                     flags = fctx.flags()
                     mctx = context.memfilectx(
                         repo,

--- a/tests/test-copies-in-changeset.t	Fri Mar 18 21:37:22 2022 -0700
+++ b/tests/test-copies-in-changeset.t	Fri Mar 18 21:15:54 2022 -0700
@@ -324,7 +324,7 @@
   $ hg ci --amend -m 'new description'
   saved backup bundle to $TESTTMP/repo/.hg/strip-backup/*-*-amend.hg (glob)
   $ hg showcopies
-  a -> l (no-extra !)
+  a -> l
   $ cd ..
 
 Test rebasing a commit with copy information