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

Hi,

When using sub-directory entries in .gitignore, "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?

Thanks.