commitctx: create the ChangingFiles object directly in the various case

No need to compute all data then create the object, we can create it early and
directly store data in it. We start simple by moving create higher in the
function, but the end goal is to eventually move the creation inside the
`_process_files` function to take advantage of the object there.

mercurial/commit.py

@@ -115,36 +115,36 b' def _prepare_files(tr, ctx, error=False,'
 
     writechangesetcopy, writefilecopymeta = _write_copy_meta(repo)
 
-    filesadded, filesremoved = None, None
     if ctx.manifestnode():
         # reuse an existing manifest revision
         repo.ui.debug(b'reusing known manifest\n')
         mn = ctx.manifestnode()
-        touched = ctx.files()
+        files = metadata.ChangingFiles()
+        files.update_touched(ctx.files())
         if writechangesetcopy:
-            filesadded = ctx.filesadded()
-            filesremoved = ctx.filesremoved()
+            files.update_added(ctx.filesadded())
+            files.update_removed(ctx.filesremoved())
     elif not ctx.files():
         repo.ui.debug(b'reusing manifest from p1 (no file change)\n')
         mn = p1.manifestnode()
-        touched = []
+        files = metadata.ChangingFiles()
     else:
-        r = _process_files(tr, ctx, error=error)
-        mn, touched, filesadded, filesremoved = r
+        mn, touched, added, removed = _process_files(tr, ctx, error=error)
+        files = metadata.ChangingFiles()
+        files.update_touched(touched)
+        if added:
+            files.update_added(added)
+        if removed:
+            files.update_removed(removed)
 
     if origctx and origctx.manifestnode() == mn:
-        touched = origctx.files()
+        origfiles = origctx.files()
+        assert files.touched.issubset(origfiles)
+        files.update_touched(origfiles)
 
-    files = metadata.ChangingFiles()
-    if touched:
-        files.update_touched(touched)
     if writechangesetcopy:
         files.update_copies_from_p1(ctx.p1copies())
         files.update_copies_from_p2(ctx.p2copies())
-    if filesadded:
-        files.update_added(filesadded)
-    if filesremoved:
-        files.update_removed(filesremoved)
 
     return mn, files