Git: Prompt for unchanged file after custom mergetool fails

To reproduce, create a conflict and configure a custom mergetool that does
nothing:

git config mergetool.my.cmd true
git config merge.tool my

Then execute merge tool from Qt Creator. The merge tool process hangs
while asking how to handle the unchanged file, and can only be killed
externally.

Change-Id: I4d2a91061bd7549e308f6a25f106e6a7afc5cadb
Reviewed-by: André Hartmann <aha_1980@gmx.de>

src/plugins/git/mergetool.h

@@ -64,6 +64,7 @@ public:
     };
 
 private:
+    void prompt(const QString &title, const QString &question);
     void readData();
     void done();