Improve i18n_tool.py list-translators, update-from-weblate
Improve i18n_tool.py list-translators, update-from-weblate

Allow the specification of a commit whose timestamp will be used as
the starting point for gathering translator credits, instead of
starting from the last commit that says "l10n: sync". That breaks
badly if there are any source string changes during the release cycle
that require another sync, so instead, just examine all the commits
since the time of the specified revision, defaulting to the last
release tag.

Also, instead of always gathering translator contributions up to the
tip of i18n/i18n for list-translators and update-from-weblate, allow
the specification of a commit. This is intended to allow verification
of these functions' results; release management tasks should use the
default target of the i18n/i18n branch tip.

conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop