filter-branch: handle "disappearing tree" case correctly in subdir filter

filter-branch: handle "disappearing tree" case correctly in subdir filter

The subdirectory filter had a bug to notice that the commit in question
did not have anything in the path-limited part of the tree.  $commit:$path
does not name an empty tree when $path does not appear in $commit.

This should fix it.  The additional test in t7003 is originally from Kevin
Ballard but with fixups.

Signed-off-by: Junio C Hamano <gitster@pobox.com>