Fixed working directory/index mismatch

Hi again!
Given a working directory where the following changes have been made per `git status -sb`:

    ## master
    M  1_index_modified.md
    D  2_index_deleted.md
     M 3_modified.md
     D 4_deleted.md
    A  5_index_new.md
    ?? 6_new.md

and utilizing posh-git-sh via `PROMPT_COMMAND='__git_ps1 "\u@\h:\w" "\\\$ "'` in ~/.bashrc, the following branch information is shown:

    [master +1 ~2 -2 | +2 ~0 -0]

versus the expected:

    [master +1 ~1 -1 | +1 ~1 -1]

Because `git status -s` puts the status for the working directory in a second column, filling the first with a space if the given file has no indexed changes yet, parsing out the index status incorrectly includes some working directory changes.

On my machine, replacing `IFS=" "` with `IFS="\n"` solves this. Pull request to match. Please try the change in your environment before accepting; it works for me on OS X 10.8 and Arch Linux, but I haven't tried it anywhere else, so YMMV.