sourceview: Fix text-wrapping reset from commit dialog (bgo#741620)

Because we set the wrap-mode forcefully in the commit dialog's
sourceview, the gsettings binding was overriding the actual set
preference.
patchdialog: Encode created patches to UTF-8

An alternative approach would be to reuse the encoding of the original
files. If we did that, we'd need to handle this better, including
adding a confirmation dialog and fallback. In the end, this is sane and
probably no one cares that much.