amend: add a test for a simplified version of issue6157

Differential Revision: https://phab.mercurial-scm.org/D6666

tests/test-amend.t

@@ -451,3 +451,20 b' Bad combination of date options:'
   [255]
 
   $ cd ..
+
+Corner case of amend from issue6157:
+- working copy parent has a change to file `a`
+- working copy has the inverse change
+- we amend the working copy parent for files other than `a`
+hg includes the changes to `a` anyway.
+
+  $ hg init 6157; cd 6157
+  $ echo a > a; echo b > b; hg commit -qAm_
+  $ echo a2 > a; hg commit -qm_
+  $ hg diff --stat -c .
+   a |  2 +-
+   1 files changed, 1 insertions(+), 1 deletions(-)
+  $ echo a > a; echo b2 > b; hg amend -q b
+  $ hg diff --stat -c .
+   b |  2 +-
+   1 files changed, 1 insertions(+), 1 deletions(-)