Git: Introduce StashGuard

Offers the user to stash changes (if relevant), stores
the results and pops the stash when it goes out of scope
(unless disabled)

Change-Id: Ibc0d2a5d3e3c953062fb17ecba903ca814524837
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/git/gitplugin.h

@@ -118,7 +118,6 @@ private slots:
     void resetRepository();
     void startRevertCommit();
     void startCherryPickCommit();
-    void startRevertOrCherryPick(bool isRevert);
     void stageFile();
     void unstageFile();
     void gitkForCurrentFile();