Git: Add m_ prefix to StashGuard members

Change-Id: Ice9f33e986688e8d2c5250435a30cd7cb1ac3b28
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Git: Only execute stash if there are changes

...even when NoPrompt is specified

Change-Id: I735133e390403a0f8f6aa573d6f9c63cf0784187
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>