tests/test-dispatch.py.out

rust-index: use a `BitVec` instead of plain `Vec` for heads computation

The `Vec` method uses one byte per revision, this uses 1 per 8 revisions,
which improves our memory footprint. For large graphs (10+ millions), this
can make a measurable difference server-side.

I have seen no measurable impact on execution speed.

running: init test1
result: 0
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: 0
running: commit -m commit2 -d 2000-01-02 foo
result: 0
running: log -r 0
changeset:   0:0e4634943879
user:        test
date:        Sat Jan 01 00:00:00 2000 +0000
summary:     commit1

result: 0
running: log -r tip
changeset:   1:45589e459b2e
tag:         tip
user:        test
date:        Sun Jan 02 00:00:00 2000 +0000
summary:     commit2

result: 0