[RFC] support for plugins to implement their own log formatters

[Erik Bågfors]

> A blackbox test would be sufficient. You make 2 commits, and then you
> check the output of "bzr log --log-format=XXX".
> Look in "bzrlib/tests/test_log.py" it should give a basic idea. (we
> should be testing for --line, and --short).
>
> But mostly I want you to write a test that forces the code with the bug
> to be run. So that a bug doesn't show up there again.

I've done that now. I am simply checking the output of bzr log/missing
by counting how many lines they output, since "short" and "line"
outputs a different number of lines.  Hope this is ok.

The tests triggers the bug which you can test by reverting one
revision. (oops, just found a typo, revert two now).

I've merged jam-integration into this branch as well, to get blackbox
tests to run.

/Erik