Git: Fix branch expansion on show before the diff display is done

The description is shown first (with the <Expand> placeholder), and the
diff parsing can be long. If <Expand> is clicked during parsing, it fails
because the base directory is not set yet.

Change-Id: Ib5e839b292c32091c726f24eb643d0fa5a142cb7
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/diffeditor/diffeditordocument.h

@@ -60,6 +60,7 @@ public:
                       const QString &startupFile = QString());
     QList<FileData> diffFiles() const;
     QString baseDirectory() const;
+    void setBaseDirectory(const QString &directory);
     QString startupFile() const;
 
     void setDescription(const QString &description);