In the description field of the commit editor, class
VCSBaseSubmitEditor provides completion of file names and C++
entities (based on QtCreator's internal C++ code model).

Change-Id: Ie5323714dbf6f6e635953dfbb35596201d86fc37
Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>
Reviewed-by: Hugues Delorme <delorme.hugues@fougsys.fr>