VCS: Fix display of diff/show commands in progress bar

In 6ae0e8d2c7 we added 2 arguments that
precede the diff sub-command (-c diff.color=false).

When display name is not set, ShellCommand uses the binary followed by the
first argument, so it now showed "Git -c" instead of "Git Diff".

Change-Id: Ie584837bd87fd529e0cd015025fc3bf1a69d085c
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Xiaofeng Wang <wasphin@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/vcsbase/vcsbasediffeditorcontroller.h

@@ -53,6 +53,7 @@ protected:
     QString workingDirectory() const;
     void setStartupFile(const QString &startupFile);
     QString startupFile() const;
+    void setDisplayName(const QString &displayName);
 
 private:
     friend class VcsBaseDiffEditorControllerPrivate;