Avoid spawning console windows when running from .pyw

By adding `shell=True,` to the list of Popen parameters, we avoid spawning console windows when scripts call this method from a windowless (.pyw) Python script.
Merge branch 'patch-1' of https://github.com/boppreh/GitPython into boppreh-patch-1

Adjusted patch to only run git command in shell mode on windows.
Conflicts:
	git/cmd.py
Merge branch 'boppreh-patch-1' into 0.3