ancestor.missingancestors: don't discard from basesvisit
We only actually care about whether revsvisit is empty, so we can let
basesvisit grow to arbitrary size.
It turns out that this actually helps performance. For a large repo with
hundreds of thousands of commits, hg perfrevset 'only(0, tip)' (basically the
worst case, involving a full DAG traversal) goes from 1.63 seconds to 1.50. hg
perfrevset 'only(tip, 0)' remains unchanged at 1.98 seconds.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present