clfilter: handle non contiguous iteration in `revlov.headrevs`
This prepares changelog level filtering. We can't assume that any revision can
be heads because filtered revisions need to be excluded.
New algorithm:
- All revisions now start as "non heads",
- every revision we iterate over is made candidate head,
- parents of iterated revisions are definitely not head.
Filtered revisions are never iterated over and never considered as candidate
head.
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1