Move the 'filenames' iterator to the next filename.
If 'filenames' is already pointing at the last filename then the
iterator will be moved to a point just beyond that last filename,
(where notmuch_filenames_valid will return FALSE and
notmuch_filenames_get will return NULL).
It is acceptable to pass NULL for 'filenames', in which case this
function will do nothing.
Move the 'filenames' iterator to the next filename.
If 'filenames' is already pointing at the last filename then the iterator will be moved to a point just beyond that last filename, (where notmuch_filenames_valid will return FALSE and notmuch_filenames_get will return NULL).
It is acceptable to pass NULL for 'filenames', in which case this function will do nothing.