commitextras: work nicely with other extensions
Before this change, it doesn't add these extra fields when loaded
alongside another extension that does a bunch of things, including
wrapping commit.
I did not investigate exactly why, but
- the documentation of extensions.wrapfunction says to use subclassing
to play nicely with other extensions
- using subclassing does make commitextras work when loaded alongside
my other extension
Differential Revision: https://phab.mercurial-scm.org/D4404