relnotes: enable extension when running releasenotes command
Last night, when I was working on different machine, the test worked, now on a
different machine it does not. Looks like we should make sure releasenotes
extension is loaded.
Differential Revision: https://phab.mercurial-scm.org/D4425
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
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