Bug when moving submodules (I think?)

I tried to make a script to repro this from scratch but ran into other issues, which may or may not be a bug. I'll put that at the end.

To repro all you have to do is: 

 git checkout git://github.com/frioux/dotfiles
 git reset --hard 92c85161ceec9e52b0b2d2de893ba11f49c80198
 git mv zsh .zsh

(sha included so this email continues to be valid in the future)

You can now see that .git/index.lock has been left behind. On a non-fresh checkout (I'm not sure why my old checkout is special) I get the following error:

 git: builtin/mv.c:248: cmd_mv: Assertion `pos >= 0' failed.

I assumed this was just related to moving submodules that are in subdirectories, but when I do that from a fresh repo I get a different error.

 mkdir -p test/a test/b
 cd test/a
 git init
 touch a.txt
 git add a.txt
 git ci -m 'initial commit'
 cd ../b
 git init
 mkdir c
 touch c/c.txt
 git submodule add ../a c/a
 git ci -m 'initial commit'
 git mv c d
 git status

And the error:

 fatal: Could not chdir to '../../../../c/a': No such file or directory
 fatal: 'git status --porcelain' failed in submodule d/a

I am using git v1.8.5.1 built from source on the latest ubuntu. If there is anything else I can do to help repro this please do not hesitate to ask.