Merge branch 'pw/sequencer-recover-from-unlockable-index' into maint

The sequencer machinery (used by "git cherry-pick A..B", and "git rebase -i", among other things) would have lost a commit if stopped due to an unlockable index file, which has been fixed. * pw/sequencer-recover-from-unlockable-index: sequencer: reschedule pick if index can't be locked