changegroup: rename "revlog" variables
"revlog" shadows the module import. But more importantly, changegroup
generation should be storage agnostic and not assume the existence
of revlogs. Let's rename the thing providing revision storage to
"store" to reflect this ideal property.
Differential Revision: https://phab.mercurial-scm.org/D4073
test-convert-git.t
test-subrepo-git.t
test-patchbomb-tls.t