emacs: create patch filename from subject for inline patch fake parts
Use the mail subject line for creating a descriptive filename for the wash
generated inline patch fake parts. The names are similar to the ones
created by 'git format-patch'.
If the user has notmuch-wash-convert-inline-patch-to-part hook enabled in
notmuch-show-insert-text/plain-hook, this will change the old default
filename of "inline patch" in fake parts: