Improve diff view: spaces and underscores

This diff:

https://ncatlab.org/nlab/revision/diff/Alexander+Grothendieck/35

has two problems:

- it shows the underscore before `_Récoltes` and `_Reminiscences`
  (when it should not)
- it eats the space between "of" and "Sasha" (in the line
  that has the change Reminiscenes -> Reminiscences)