Ediff User's Manual
7.11 Customizing the Mode Line
When Ediff is running, the mode line of `Ediff Control Panel'
buffer shows the current difference number and the total number of
difference regions in the two files.
The mode line of the buffers being compared displays the type of the
buffer (`A:', `B:', or `C:') and (usually) the file name.
Ediff tries to be intelligent in choosing the mode line buffer
identification. In particular, it works well with the
`uniquify.el' and `mode-line.el' packages (which improve on
the default way in which Emacs displays buffer identification). If you
don't like the way Ediff changes the mode line, you can use
ediff-prepare-buffer-hook to modify the mode line.