From the commit message:

    emacs: add notmuch-before- and notmuch-after-tag-hook
    
    This patch adds hooks that are run before/after messages are tagged
    from the emacs interface.  In order to implement this and to avoid
    having hooks parse all the arguments to the notmuch binary again, I
    created a `notmuch-tag' function that other modules should use instead
    of running (notmuch-call-notmuch-process "tag" ...) directly.

I wasn't sure about adding the notmuch-tag function, but I think it is a
bit cleaner than calling notmuch-call-notmuch-process directly in so
many places anyway.

Of course, any comments or criticism is appreciated.

-- Daniel