stack: rename stack.revs into stack.indexedrevs

They are accessible by an index, and are sorted. But they don't include all
revisions for show in showstack().
2 jobs for topic/default/stack-things-2 in 12 minutes and 13 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #29193
flake8

00:00:16

passed #29194
tests

00:12:13