revlog: return lazy set from findcommonmissing

When computing the commonmissing, it greedily computes the entire set
immediately. On a large repo where the majority of history is irrelevant, this
causes a significant slow down.

Replacing it with a lazy set makes amend go from 11 seconds to 8.7 seconds.
strip: add faster revlog strip computation

The previous revlog strip computation would walk every rev in the revlog, from
the bottom to the top.  Since we're usually stripping only the top few revs of
the revlog, this was needlessly expensive on large repos.

The new algorithm walks the exact number of revs that will be stripped, thus
making the operation not dependent on the number of revs in the repo.

This makes amend on a large repo go from 8.7 seconds to 6 seconds.
manifest: increase lrucache from 3 to 4

During a commit amend there are 4 manifests being handled:

- original commit
- temporary commit
- amended commit
- merge base

This causes a manifest cache miss which hurts perf on large repos. On a large
repo, this fix causes amend to go from 6 seconds to 5.5 seconds.