notmuch.el: Insert a newline if the last line of a part is missing one.

I think there's a GMime bug that we're getting parts decoded without a
final newline (the encoded parts seem to have them just fine). We can
workaround the bug easily enough by finding a part-closing delimiter
that is not at the beginning of a line, and if so, just insert a
newline.

Without this, the one-line-summary of the next message would continue
on the same line as the last line of the previous message, (and this
would often happen for mailing-list messages where mailman would add
an extra part for its signature block).