[BUG] git clean -X behaviour when .gitignore has sub-directory entries

(this is a repost w/ a bit more context)

Hi,

Context: "git clean -X" is really helpful in my projects to mimick a "make clean" (or more precisely a "make distclean") but I believe there is a small problem with the -X option of git clean. I've validated this with other people and they agree it's a bug.

Problem: When using sub-directory entries in .gitignore, while the entry is ignored as expected, "git clean -X" doesn't clean the ignored sub-directory.

$ git init test
Initialized empty Git repository in /home/jpgariep/git/test/.git/
$ cd test/
$ mkdir -p a/b/c
$ touch a/b/c/test
$ echo '/a/b/' > .gitignore
$ git add .gitignore
$ git commit -m "Added .gitignore."
[master (root-commit) 94e2825] Added .gitignore.
  1 files changed, 1 insertions(+), 0 deletions(-)
  create mode 100644 .gitignore
$ git status
# On branch master
nothing to commit (working directory clean)
$ git clean -X -d -f
$ ls a
b

Why is b still there?

Git version: git version 1.7.2.3 (but this has been around since 1.6.something) Tested also with 1.7.3.2.g9027fa (today's HEAD of master branch)

Thanks.