See diffs when editing commit message with VIM
Note: Recent versions of hgeditor (e.g. tip's) include this functionality already. (i.e. you don't have to do anything!)
Make a private copy of the hgeditor script provided with Mercurial and replace the call to the editor with following command:
vim "+e $HGTMP/diff" '+set buftype=help filetype=diff' "+vsplit $HGTMP/msg" || exit $?
This will start up VIM in vertical split mode with commit message in the left pane and diff in the right pane. The buftype=help setting for diff window tells vim to exit when all other windows are closed, so when you write and quit the log with :x (:wq - they are equivalent), vim exits. If you have syntax highlight set up, the diff will be properly highlighted.
This setting is suitable for wide terminals. If you have a narrow terminal, you may want to replace the +vsplit above with +split or add nowrap to the +set.