automation: install latest Python versions This required bumping the pyenv commit so the new versions are available. Differential Revision: https://phab.mercurial-scm.org/D6713

upgrade: introduce the internal code for revlog cloning selection For now we still clone every single revlogs but all the selection mechanism is now in place in the lower layer. The next changesets will introduce the user interface part of the selection.

relnotes: copy "next" to "5.1" and clear "next" To avoid merge conflicts, we want to avoid modifying the file on multiple branches in parallel. This patch is therefore meant to be applied to the stable branch and then quickly be merged to default (at least before edits are made to relnotes/next there). Another option would have been to copy the file on the stable branch and to clear it on the default branch. However, that still results in conflicts if the copy is edited on the stable branch (Mercurial would try to apply the changes from the default branch to it). We could also delete the file in one commit and recreate it in another commit. However, Mercurial is quite inconsistent in what it considers a break in history (see test-copies-unrelated.t), so I'd like to avoid that. Differential Revision: https://phab.mercurial-scm.org/D6705

upgrade: introduce a _copyrevlog method This function copies a revlog from the old store to the new one, without re-adding all deltas manually. This will eventually save a lot of time when some revlog does not needs any conversions. Code actually using this will be introduced in later changesets.

upgrade: rename `_copyrevlogs` to `_clonerevlogs` The underlying revlog method is named `clone`, keeping the naming consistent seems clearer. This is motivated to clarify the difference with an (upcoming) function that simply copy revlog files as is.

upgrade: walk the source store file only once I don't expect this to have a significant performance impact, but it seems simpler and saner to do the operation only once and to keep the result around.