docs: change `divergent` references to `content-divergent` Per https://www.mercurial-scm.org/wiki/CEDVocabulary

docs: change `troubled` references to `unstable` Per https://www.mercurial-scm.org/wiki/CEDVocabulary

docs: change `unstable` references to `orphan` Per https://www.mercurial-scm.org/wiki/CEDVocabulary

doc: correct a typo

docs: change `troubles` references to `instability` Per https://www.mercurial-scm.org/wiki/CEDVocabulary. There are a couple instances of 'troublesome' which probably need a word, as deriviatives of 'instability' didn't really fit. At least some of these are in the unlinked obs-terms.rst page.

docs: change `precursors` references to `predecessors` Per https://www.mercurial-scm.org/wiki/CEDVocabulary

docs: import figure diagrams with inkscape 0.92 No changes to the content here. This simply accepts the default option ("This file contains digital artwork for screen display") on the prompt when first opened, which is trying to upgrade from 90DPI to 96DPI.

messages: standardize on "working copy is now at" including "is" It seemed that we mostly used "is", but `hg next` and `hg prune` could print it without the "is".

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).