range-diff: internally force `diff.noprefix=true`

range-diff: internally force `diff.noprefix=true`

When parsing the diffs, `range-diff` expects to see the prefixes `a/`
and `b/` in the diff headers.

These prefixes can be forced off via the config setting
`diff.noprefix=true`. As `range-diff` is not prepared for that
situation, this will cause a segmentation fault.

Let's avoid that by passing the `--no-prefix` option to the `git log`
process that generates the diffs that `range-diff` wants to parse.
And of course expect the output to have no prefixes, then.

Reported-by: Michal Suchánek <msuchanek@suse.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>