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 T1155