Mercurial 5.7

The 5.7.1 tag got published today. Seems reasonable to include in the next minor Heptapod version.

Task list (next step would be to make an issue template out of it):

• bump in default branch of heptapod/ci-images/mercurial. Subsequent checks:
  • hg-evolve version
  • versions in stable branches
  • results of downstream (py-heptapod etc) pipeline triggered by publication of the change
• main repo: bump Mercurial and hg-evolve in heptapod/heptapod, with [skip ci], as a draft in heptapod branch.
• rebuild heptapod/ci-images/heptapod-base (it uses head of heptapod branch of main repo). Subsequent checks:
  • hg-evolve version
  • heptapod/ci-images/heptapod-gitaly rebuilt triggered by heptapod/ci-images/heptapod-base pipeline
• trigger CI on main repo: remove [skip ci] and repush on heptapod/heptapod, launch pipeline manually if needed. Subsequent checks:
  • mercurial and evolve versions (look for hpd-versions in any of the RSpec jobs logs)
• merge in release branch of main repo, run pipeline, including manual tarball job
• make intermediate Omnibus release (using manual tarball and SHA256 visible in main repo tarball job)
• publish everything if ok up to functional tests