bug#50104: 28.0.50; 'M-o' is bound in diff-mode-map

[Juri Linkov]

> 'M-o' is bound to 'diff-goto-source' in 'diff-mode-map'.  I guess this
> binding should be removed, to be in line with the freeing up of 'M-o'
> elsewhere.  Otherwise, a global binding to 'M-o' is overridden in diff
> buffers.

The standard key to display a buffer in another window is 'C-o', not 'M-o',
like is used in list-buffers, dired, compilation, grep, occur, etc.,
so maybe diff-mode should use the same.

> Note also that there are several other bindings for that command in diff
> buffers: 'RET', 'C-c C-c', etc.
>
> (While we're at this, I'll express my opinion that, apart from leaving
> 'M-o' reserved for the user to customize, 'other-window' would the only
> sensible use for that key.)

For the long time, the bigger problem was to get rid of remaining 'M-s'
keybindings.  For example, it's impossible to use word search 'M-s M-w'
in log-edit buffers (where 'M-s' is bound to log-edit-comment-search-forward)
and in some other modes.