shortest: extract function for checking if a prefix is a revnum
Much of isvalid() was about testing if a prefix is a valid revnum. I
want to reuse that soon, so let's move it out.
There is no significant slowdown from the function call overhead.
Differential Revision: https://phab.mercurial-scm.org/D3460
<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>