Fix fd leak on git cmd.

Currently if command is called with as_proces=True, pipes for the
command will not be closed.

This change makes sure to close command file descriptors.

Conflicts:
	git/cmd.py
Ignore signal exception on AutoInterrupt destructor.

When command run as subprocess, AutoInterrupt will kill the
process on destructor. However, if process already finished,
it raise OSError exception.

This fix just ignore OSError on os.kill.

Conflicts:
	git/cmd.py