New snippets (created with yas-new-snippet) may not yet be associated
with a file when it is passed to the functions in yas-prompt-functions.
One of those functions, +snippets-prompt-private, prioritizes private
snippets over default ones, but determines which is which with
`file-in-directory-p`, which expects string arguments.
To fix this, we can safely assume that if a snippet has no associated
file, it is a private snippet.
68e2b814e7