Support diff a file from project explorer against the current file

Task-number: QTCREATORBUG-9432
Change-Id: Ie370bbffdb67dac520f392a73c1358f887157806
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/coreplugin/diffservice.h

@@ -37,6 +37,7 @@ class CORE_EXPORT DiffService
 public:
     virtual ~DiffService() {}
 
+    virtual void diffFiles(const QString &leftFileName, const QString &rightFileName) = 0;
     virtual void diffModifiedFiles(const QStringList &fileNames) = 0;
 };