Loosen regex to compensate for iskeyword

Closes #332.
Add foldenable to fugitive_diff_restore
Revert "Add foldenable to fugitive_diff_restore"

This reverts commit cbb7c3e89026472dfbc5440d78ce85975423bedd.
Encapsulate restore options for diff

This should make it easier to experiment with solutions for issues such
as #257.