swh-loader-mercurial: Fix invalid release target and add missing data
When processing hg tags and transforming them to swh releases, the target revisions were not correctly computed. Instead of pointing to a swh revision, the original hg tag changeset id was used.
That diff adds a global mapping between hg changesets and swh revisions in order to compute the correct revision target identifier when processing a mercurial tag. Missing release data (message, author, date) are also added from the associated target revision.
Related #1155 (closed)
Migrated from D395 (view on Phabricator)