keyword: clarify object of backwards compatibility

commit: write last-message.txt with no content modifications. This assumes the user will use either a sane editor (anything but Notepad) or the same editor again: even Notepad should be able to read what it previously wrote.

mail: add parseaddrlist() function for parsing many addresses at once Adds a new function addrlistencode() to mercurial.mail, like addressencode(), but accepts a list of addresses as input, and returns a list of formatted addresses. This will be used by patchbomb. The difference is that each element in the input list can contain multiple addresses (comma separated or otherwise), and are split using Python's email.Utils.getaddresses().