[BUG?] --boundary inconsistent with path limiting

Let's say I have a simple repo with three paths:

	git init -q repo
	cd repo
	for i in a b c
	do
		echo content >$i.t
		git add $i.t
		git commit -qm $i &&
		git tag $i
	done

If I ask for the top 2 commits, with the third as a boundary, I get the expected output:

  $ git log --format='%m %s' --boundary a..c
  > c
  > b
  - a

If I limit the path to "b.t", I get:

  $ git log --format='%m %s' --boundary a..c -- b.t
  > b
  - a

That makes sense to me. We omit "c" because it doesn't touch "b.t", and obviously include "b", which does. We _do_ include the boundary commit, even though it doesn't touch the path, which makes sense to me. It remains a boundary whether it touched the path or not, and without it, we get no boundary at all.

But now if I limit to "a.t", I get no output at all:

  $ git log --format='%m %s' --boundary a..c -- a.t

whereas I would have expected "- a" to show the boundary.

Is this a bug, or are my expectations wrong?

-Peff