hgeditor is a script distributed with Mercurial in the examples/ directory (installed to /usr/share/doc/mercurial/examples/hgeditor on some Linux distributions). By setting the HGEDITOR environment variable, a user may cause the hgeditor script (or a customized copy of it) to be called when files are committed. TipsAndTricks discusses several ways of customizing this script.
If an hgeditor script was not included in your Mercurial distribution, you may also find it here.