SVN: Add method to find SVN version and use it in svnDiff

We want to enforce the internal diff if possible, but that option
is only in 1.7 and later.

Task-number: QTCREATORBUG-9425
Change-Id: If89f0a7eedae51a04a5f1c40d48287a6425f7a12
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>