tests/test-ro-message.out

Change revlog.heads to walk the revision graph using revision numbers

On the kernel repo:
$ hg heads -q
           before    after
RevlogNG    1.11     0.52
Revlogv0    0.80     0.69

Since the current code for tags has to find all the heads of the repo,
this also helps there:
$ hg tags
           before    after
RevlogNG    2.35     1.76
Revlogv0    2.04     1.90

1 files updated, 0 files merged, 0 files removed, 0 files unresolved
2 files updated, 0 files merged, 0 files removed, 0 files unresolved
abort: Permission denied - test-ro-message/b/vehicle