bug with partial commit and pre-commit hook updating the index

I have found many uses for the feature that lets a pre-commit hook stage changes in the index that will be included in the commit. But now I seem to have found a bug in the support for that, involving partial commits.

It seems that, after a partial commit in which the pre-commit hook stages a modification of a file, the index is is left without that staged change. This only occurs with "git commit $file", not "git commit -a".

joey@darkstar:~/tmp/a>ls
joey@darkstar:~/tmp/a>date > foo
joey@darkstar:~/tmp/a>git add foo
joey@darkstar:~/tmp/a>git commit -m "added regular file foo"
[master 79d0f1d] added regular file foo
 1 file changed, 1 insertion(+)
 create mode 100644 foo
joey@darkstar:~/tmp/a>mv ~/pre-commit  .git/hooks/
joey@darkstar:~/tmp/a>cat > .git/hooks/pre-commit
#!/bin/sh
ln -vsf /etc/passwd foo
git add foo
joey@darkstar:~/tmp/a>date > foo
joey@darkstar:~/tmp/a>git commit foo -m update
‘foo’ -> ‘/etc/passwd’
[master efa9f67] update
 1 file changed, 1 insertion(+), 1 deletion(-)
 rewrite foo (100%)
 mode change 100644 => 120000

So, the pre-commit hook replaced file foo with a symlink, and staged it, and we can see from the commit summary that was correctly included in the commit. But, look here:

joey@darkstar:~/tmp/a>git status
On branch master
Changes to be committed:
  (use "git reset HEAD <file>..." to unstage)

	typechange: foo

Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	typechange: foo

builtin/commit.c has a long comment that talks about a false index which is set up and used during a partial commit. The pre-commit hook is run using this false index, and the commit is generated from it. I guess the bug involves the real index not being updated afterwards to reflect the changes made to the false index.