Move ChangeLog to ChangeLog.old/ChangeLog.19
We no longer maintain a manually-written ChangeLog file:
<https://sourceware.org/ml/libc-alpha/2019-09/msg00333.html>
<https://sourceware.org/ml/libc-alpha/2019-10/msg00131.html>
Instead the release manager is expected to generate a ChangeLog-like
file using scripts/gitlog_to_changelog.py. For further details,
see commit
f2144b7874b23be7c7eb184ec601633ec6fa8fac ("Script to
generate ChangeLog-like output from git log").
Reviewed-by: Carlos O'Donell <carlos@redhat.com>