perf: support measuring bdiff for all changeset related data
The --all argument changes the behavior of `perfbdiff` to pull
in fulltext revision pairs for all changes related to a changeset.
The p1 and p2 manifests will be bdiffed against current. Every file
that changed between p1 and current will have its file revisions
loaded and bdiffed.
This mode of operation effectively measured the bdiff time required
for `hg commit`.
perf: prepare to handle multiple pairs in perfbdiff
Before, we only supported benchmarking a single pair of texts
with bdiff. We want to enable feeding larger corpora into this
benchmark. So rewrite the code to support that.