rust-revlog: move check for nodemap requirement to caller
It's good for both making `Revlog` testable and reusable to have it
not depend on the higher-level `Repo` type. This patch is one step in
towards that. Additionally, this change in particular gives the
callers more control over when to use a nodemap.
Differential Revision: https://phab.mercurial-scm.org/D12546
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL