[PATCH 2 of 8] diff: Move diffline to patch module

[bisho (Guillermo)]

> > The move of this function seems unrelated to the purpose of the patch.
> Probably because it didn't need to be a top-level function.
 
> > Also, why are we making it public?
> It's not public, it's a function nested in another.

Exactly, is not public, and I moved it within this patch because diffline
and that function follow the same behavior, they are helpers of trydiff,
so makes sense to move them together and nest them under trydiff.