scmutil: fix requires-file isalnum() check on first byte
Slice instead of subscript to get a bytes from a bytes.
# skip-blame just a py3 slice-instead-of-subscript change
Differential Revision: https://phab.mercurial-scm.org/D2341
<table class="slogEntry parity{parity}">
<tr>
<td class="age">{date|rfc822date}</td>
<td class="author">{author|person}</td>
<td class="node"><a href="{url|urlescape}rev/{node|short}{sessionvars%urlparameter}">{desc|strip|firstline|escape|nonempty}</a></td>
</tr>
</table>