mergetools: refine vimdiff warning message
We explicitly redraw before echoing the message so that it simply
displays at the bottom of the window. Also simplifies the message
printing by using 'echomsg' (which uses 'echohl' internally) and adds
the names of the software involved for improved Googleability.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.