emacs: Handle all face forms when combining faces

emacs: Handle all face forms when combining faces

Previously, notmuch-combine-face-text-property assumed that any
existing face properties of the modified text were already in face
list form.  This was true as long as it was the only function
manipulating faces (since it always produced a list form face), but if
anything else has manipulated the face, it was more likely to be
either a face name or a face plist.  It also didn't correctly handle
face lists as arguments, even though the doc string claimed it did.

This patch fixes notmuch-combine-face-text-property to handle all face
forms correctly by canonicalizing both the argument face and the
existing faces into list form.  This also means we can set the face to
a simpler non-list form if there's no existing face.