It seems like the point of hg log --follow-first is to show the history of what
was in the central repository.  However, hg log --follow-first on a file doesn't
seem particularly useful, because it doesn't show anything at all when a change
to the file was introduced in a change that needed to be merged in order to be
pushed to that repository.  It seems like hg log --follow-first, when given a
file or directory argument (which causes the log to be subsetted to show only
the changes that are relevant to that file), ought to show the merge commits
that introduce changes to that file along the un-subsetted history that would be
shown by an hg log --follow-first on the whole repository.

To give a more concrete example, see the attached log.  I think the output of
"hg log --follow-first foo" near the end of the log should show changesets 0 and
3.  It currently shows only changeset 0.  (hg log --follow-first on the whole
repository should, and I think does, show changesets 0, 1, and 3.)