cmdutil: changeset_printer: use methods of filectx/changectx.
This allows extensions that modify changeset metadata (e.g.
description) by overriding methods of changectx to get consistent
behavior from all log-like commands, regardless of whether templates
or styles are used. Without this, overriding changectx methods works
if you use styles or templates, but not with default log format.
This meant adding filectx.extra() for consistency with changectx.
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "hgk=" >> $HGRCPATH
hg init repo
cd repo
echo a > a
hg ci -Am adda
hg debug-cat-file commit 0