Re: default file for diff-backup

[Alfred M. Szmidt]

   > The idea is to supply the current file name of the buffer if you just
   > hit enter, i.e exact same behaviour as for M-x diff RET.

   I get that already (even though it doesn't explicitly say so in the prompt). 
 Just like, say, `copy-file'.

There are quite a few cases where the default could be improved for
the "RET" case, copy-file, rename-file, delete-file, ...