rpm: Use %config(noreplace) for the config file (#932)

Currently, when upgrading the RPM, if the upstream `zrepl.yml` is
changed in the update, the user's `zrepl.yml` will be renamed to
`zrepl.yml.rpmsave` and the new `zrepl.yml` is installed in its place.

This commit changes the behavior so that the user's config remains in
place and the new upstream config is installed to `zrepl.yml.rpmnew`
instead.

---

This was never a problem before because
`packaging/systemd-default-zrepl.yml` was the same since the initial
release. RPM leaves edited config files alone with the upstream config
file doesn't change. However, v0.7.0 changed the file with:
9c63736489. So with this update, RPM
detects an upstream config change during the update and moves the user's
config according to `%config`'s rules
(https://www.cl.cam.ac.uk/~jw35/docs/rpm_config.html).
`%config(noreplace)` prevents this from happening.

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

packaging/rpm/zrepl.spec

@@ -69,7 +69,7 @@ cp -a   internal/config/samples                         %{buildroot}%{_datadir}/
 %{_bindir}/zrepl
 %config %{_unitdir}/zrepl.service
 %dir %{_sysconfdir}/zrepl
-%config %{_sysconfdir}/zrepl/zrepl.yml
+%config(noreplace) %{_sysconfdir}/zrepl/zrepl.yml
 %{_datadir}/zsh/site-functions/_zrepl
 %{_datadir}/bash-completion/completions/zrepl
 %{_datadir}/doc/zrepl