revset: use `subset &` in `divergent`
This takes advantage of the `fullreposet` smartness.
revset #0: divergent()
0) wall 0.002047 comb 0.000000 user 0.000000 sys 0.000000 (best of 813)
1) wall 0.000052 comb 0.000000 user 0.000000 sys 0.000000 (best of 22757)
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