hints on splitting out part of a branch

[Wichmann, Mats D]

I recall there has been discussion a few time in the past about
approaches for moving a subset of a branch out into its own
branch, but retaining some of the original context (commit history,
at least as relates to the subset).  And I can't find any of that...

We've got a branch which is watched by a job submission task
which sends it to buildbot for rebuilding when there's a commit.
One subdirectory of this branch has over the years grown apart
so it no longer has a connection with the rest of it; and it does
not contribute to the package that is built from the rest of the
branch so it's irritating when a change in this area triggers a
rebuild, needlessly (actually, a whole cascading set of events
which is a bit expensive).  People would prefer to see it split
off, but not to have all of the past history lost.

Thoughts?