patman: Add a tag for when a patch gets added to a series

patman: Add a tag for when a patch gets added to a series

When a patch is added to a series after the initial version, there are no
changes to note except that it is new. This is typically done to suppress
the "(no changes in vN)" message. It's also nice to add a change to the
cover letter so reviewers know there is an additional patch. Add a tag to
automate this process a bit.

There are two nits with the current approach:

- It favors '-' as a bullet point, but some people may prefer '*' (or
  something else)
- Tags (e.g. 'patman: ' in 'patman: foo bar') are not stripped. They are
  probably just noise in most series, but they may be useful for treewide
  series to distinguish 'gpio: frobnicate' from 'reset: frobnicate', so
  I've left them in.

Suggestions for the above appreciated.

Suggested-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>