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.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program