Fix handle is invalid error on windows

The package doesn't currently seem to work on windows since it throws this error:
```
GitCommitMsg - Command: echo off && for /f "tokens=1" %a in ( '"git blame "C:\proj\test.txt" -L 23,23 --root -s -l"') do git show --name-status "%a"
Exception in thread Thread-119:
Traceback (most recent call last):
  File ".\threading.py", line 532, in __bootstrap_inner
  File ".\GitCommitMsg.py", line 24, in run
  File ".\subprocess.py", line 626, in __init__
  File ".\subprocess.py", line 734, in _get_handles
  File ".\subprocess.py", line 773, in _make_inheritable
WindowsError: [Error 6] The handle is invalid
```
this seems to be due to an issue with python on windows which is documented here: http://bugs.python.org/issue3905
It's basically caused if you don't tell subprocess to PIPE all of the
handles so the fix is to specify stdin = subprocess.PIPE