evolve: move more of the progress-related variables together

evolve: delete dead calculation of number of troubled revisions The "count" variable is recalculated in the regular (no --continue) code path and it isn't used anywhere else (it used to be set to 1 in the --continue code path, but that now has its own progress counting).

evolve: drop nested locks and transactions All callers of _solveone() now take the locks and start the transactions, so there's no need to do that anymore.

next: start transaction before calling _solveone() All other callers do this, and I'm about to make _solveone() not create a transaction.

evolve: use single transaction also when continuing

evolve: run in a single transaction This speeds up `hg evolve --all` of a stack of 24 commits in the hg repo from 11.5s to 3.7s (3x). As can be seen in the tests, it also avoid some warnings about transiently unstable commits.

evolve: extract function for solving one revision, including merges Will make the next patch simpler.

evolve: mention that --all is the default behavior now

evolve: mention that not all successful operations currently update wdir

evolve: add progress support for --continue The progress starts where it left off when the previous command ran into merge conflicts.

evolve: reduce scope of progress-related variables

evolve: move progress-clearing out of _cleanup() The progress is only used with --all or --rev, so there's no need to clean it in any other case. It's therefore easier to clean it specifically in that case.

evolve: stop passing no-op "progresscb" into continueevolve() The "progresscb" doesn't do anything in the --continue case, so there's no need to pass it into continueevolve(). This lets us remove the --no-all that was added to the test as a workaround.

evolve: _solveunstable() update progress only once There's no need to draw it, then possibly write text over it (with --verbose), and then draw it again.

evolve: don't update progress just before clearing it _cleanup() will clear the progress, so there's little reason to update it just before (except for progress.debug=yes).