Check also property-modified files in hook script check-mime-type.pl

* contrib/hook-scripts/check-mime-type.pl:
  (proplist) add property-modified files for checks

Patch by: Matthias Bücher <maddes+subversion@maddes.net>
Suggested by: Leo Davis [1]

[1] https://mail-archives.apache.org/mod_mbox/subversion-dev/201403.mbox/%3C1576503.m6XB7udPXQ@hurry.speechfxinc.com%3E