VCS[git]: Add 'Revert this chunk' context menu option to diff view.

Implement in git. Add infrastructure to revert single chhunks
by using patch -R. Currently only implemented in git since
only that has functionality to re-run diff.

Rubber-stamped-by: hunger <tobias.hunger@nokia.com>

src/plugins/vcsbase/commonvcssettings.h

@@ -58,6 +58,8 @@ struct CommonVcsSettings
     // Executable run to graphically prompt for a SSH-password.
     QString sshPasswordPrompt;
 
+    QString patchCommand;
+
     bool lineWrap;
     int lineWrapWidth;