[BUG] index corruption with git commit -p

Hi,

I've just had a scary error:
	error: index uses $?+? extension, which we do not understand
	fatal: index file corrupt

Things were quickly recovered by deleting the index but it clearly looks to a but to me.

Here are the steps to reproduce it:
  $ git clone git://github.com/lucvoo/sparse-dev.git <somedir>
  $ cd <somedir>
  $ git co index-corruption
  $ git rm -r validation/ Documentation/
  $ git commit -m <some message> -p
  $ git status
error: index uses $?+? extension, which we do not understand
fatal: index file corrupt

The 'extension' pattern '$?+?', can vary a bit, sometimes it's just '????', but always seems 4 chars. If the commit command doesn't use the '-p' flag, there is no problem. The repository itself is not corrupted, it's only the index. It happends with git 2.18.0 and 2.17.0

-- Luc Van Oostenryck