remotefilelog: remove support for fetching packs
We've never used this at Google, and it's strictly an optimization
that indygreg and I are confident we can handle more elegantly
later. Let's rip it out to keep the surface area of the imported
remotefilelog small.
Some tests have changes because the tests for regular packs and
repacking were mixed with tests of fetching packs. I'm pretty sure
I've done the right thing here.
Differential Revision: https://phab.mercurial-scm.org/D5122
#require docutils gettext
$ $TESTDIR/check-gendoc da
checking for parse errors