We haven't needed this before since we were only using the function for text in
confirmations and menus, which is unlikely to contain tabs. We are going to use
it for patches in the staging view though, which often do.
to make it more generally usable by clients other than ConfirmationHelper, which
we will do later in this branch. Rename it to WrapViewLinesToWidth while we're
at it.
Add tests; in particular, add a sanity check that we wrap lines the same way as
gocui does. The tests that are added here are the same ones as in gocui for its
lineWrap function, but we'll extend them a bit in later commits in this branch.
