Explicitly adding ignored files in subdirectories not working

Heya,

I've been bitten by this before, is this behavior intentional? We don't even warn that the user's action was ignored; at the very least we should do that. The current behavior is just frustrating.

sverre@laptop-sverre:~/code$ git init ignoretest
Initialized empty Git repository in /home/sverre/code/ignoretest/.git/
sverre@laptop-sverre:~/code$ cd ignoretest/
sverre@laptop-sverre:~/code/ignoretest$ mkdir subdir
sverre@laptop-sverre:~/code/ignoretest$ echo "hi" >> subdir/a.txt
sverre@laptop-sverre:~/code/ignoretest$ echo "subdir" >> .gitignore
sverre@laptop-sverre:~/code/ignoretest$ git add .gitignore
sverre@laptop-sverre:~/code/ignoretest$ git commit -m "set up ignore"
[master (root-commit) 5a5a614] set up ignore
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 .gitignore
sverre@laptop-sverre:~/code/ignoretest$ git add subdir/a.txt
sverre@laptop-sverre:~/code/ignoretest$ git status
# On branch master
nothing to commit (working directory clean)