rebase: prefer --default-prefix to --{src,dst}-prefix for format-patch

rebase: prefer --default-prefix to --{src,dst}-prefix for format-patch

When git-rebase invokes format-patch, it wants to make sure we use the
normal prefixes, and are not confused by diff.noprefix or similar. When
this was added in 5b220a6876f (Add --src/dst-prefix to git-formt-patch
in git-rebase.sh, 2010-09-09), we only had --src-prefix and --dst-prefix
to do so, which requires re-specifying the prefixes we expect to see.
These days we can say what we want more directly: just use the defaults.

This is a minor cleanup that should have no behavior change, but
hopefully the result expresses more clearly what the code is trying to
accomplish.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>