2.8.0 gitignore enhancement not working as expected

Hi,

I've been testing out git 2.8.0 rc3 due to having a particular interest in this enhancement from the changelog:
" Another try to improve the ignore mechanism that lets you say "this
   is excluded" and then later say "oh, no, this part (that is a
   subset of the previous part) is not excluded".  This has still a
   known limitation, though."

Currently, in order to include ONLY the files under a/b/c I have the following in .gitignore: /* - Ignore everything !/a - Except directory "a" /a/* - Ignore all sub-directories of "a/" !/a/b - Except "a/b" /a/b/* - Ignore all sub-directories of "a/b/" !/a/b/c - Except "a/b/c"

My hope was that with this enhancement I could massively simplify this to: /* - Ignore everything !a/b/c - Except "a/b/c"

However this doesn't seem to work - instead I find that NOTHING is ignored (i.e. it's as if the 2nd line completely cancels the first).

Is this a bug or am I mis-understanding what the enhancement does? Or is this the known limitation referred to?

Thanks,

Richard