[PATCH 1 of 2] copy: document current behavior of 'hg cp --after'

[Augie Fackler]

# HG changeset patch
# User Augie Fackler <augie at google.com>
# Date 1474319683 14400
#      Mon Sep 19 17:14:43 2016 -0400
# Node ID 0ee023b61175e76d51726a51c09af6fccf1e8846
# Parent  e40343ce9c4c4819ea5669abb3447d5c031a8270
copy: document current behavior of 'hg cp --after'

I'm about to propose an output change here, but the existing behavior
was untested!

diff --git a/tests/test-copy.t b/tests/test-copy.t
--- a/tests/test-copy.t
+++ b/tests/test-copy.t
@@ -224,6 +224,9 @@ copy --after on an added file
 foo was clean:
   $ hg st -AC foo
   C foo
+Trying to copy on top of an existing file fails,
+  $ hg copy -A bar foo
+  foo: not overwriting - file exists
 but it's considered modified after a copy --after --force
   $ hg copy -Af bar foo
   $ hg st -AC foo