salvaged: record salvaged in ChangingFiles at commit time

The new code is a simple but effective way to detect this information. We might
be able to move it inside the various conditionnal above, but I want to focus
on simplicity until we have a full working stack.

It is worth noting that if we record the information in the ChangingFiles
object, it is not persisted yet. This will comes with later changesets.

Differential Revision: https://phab.mercurial-scm.org/D9120

mercurial/commit.py

@@ -140,6 +140,19 b' def _prepare_files(tr, ctx, error=False,'
         files.update_copies_from_p1(ctx.p1copies())
         files.update_copies_from_p2(ctx.p2copies())
 
+    copy_sd = ctx.repo().filecopiesmode == b'changeset-sidedata'
+    if copy_sd and len(ctx.parents()) > 1:
+        # XXX this `mergestate.read` could be duplicated with a the merge state
+        # reading in _process_files So we could refactor further to reuse it in
+        # some cases.
+        ms = mergestate.mergestate.read(repo)
+        if ms.active():
+            for fname in sorted(ms._stateextras.keys()):
+                might_removed = ms.extras(fname).get(b'merge-removal-candidate')
+                if might_removed == b'yes':
+                    if fname in ctx:
+                        files.mark_salvaged(fname)
+
     return mn, files