rebase: also restore "ui.allowemptycommit" value
It looks like this was lost when the code was converted to the
ui.configoverride() context manager in
f255b1811f5e (rebase: get rid
of ui.backupconfig, 2017-03-16). (And then the bad example was
duplicated in
228916ca12b5 (rebase: add concludememorynode(), and call
it when rebasing in-memory, 2017-12-07).)
Differential Revision: https://phab.mercurial-scm.org/D2760
<li class="parity{parity}" data-node="{node|short}">
<div class="fg">
<span class="desc">
<a href="{url|urlescape}rev/{node|short}{sessionvars%urlparameter}">{desc|strip|firstline|escape|nonempty}</a>
</span>
{alltags}
<div class="info"><span class="age">{date|rfc822date}</span>, by {author|person}</div>
</div>
</li>