tests: update test-copies-chain-merge.t to not use empty files
Merging empty files is not very interesting or realistic.
Differential Revision: https://phab.mercurial-scm.org/D9172
== New Features ==
* `hg mv -A` can now be used with `--at-rev`. It behaves just like
`hg cp -A --at-rev`, i.e. it marks the destination as a copy of the
source whether or not the source still exists (but the source must
exist in the parent revision).
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==
* `merge.update()` is now private (renamed to `_update()`). Hopefully
the higher-level functions available in the same module cover your
use cases.