completion: difftool takes both revs and files

completion: difftool takes both revs and files

'git difftool' is clearly a frontend to 'git diff' and is used in
exactly the same way, but it uses a misleadingly named completion
function __git_complete_file.  It happens to work only because it
calls __git_complete_revlist_file that completes both revs and
paths.

Change it to use __git_complete_revlist_file, just like 'git diff'.

Signed-off-by: Ramkumar Ramachandra <redacted>
Signed-off-by: Junio C Hamano <redacted>