format-patch: add format.noprefix option

format-patch: add format.noprefix option

The previous commit dropped support for diff.noprefix in format-patch.
While this will do the right thing in most cases (where sending patches
without a prefix was an accidental side effect of the sender preferring
to see their local patches without prefixes), it left no good option for
a project or workflow where you really do want to send patches without
prefixes. You'd be stuck using "--no-prefix" for every invocation.

So let's add a config option specific to format-patch that enables this
behavior. That gives people who have such a workflow a way to get what
they want, but makes it hard to accidentally trigger it.

A more backwards-compatible way of doing the transition would be to have
format.noprefix default to diff.noprefix when it's not set. But that
doesn't really help the "accidental" problem; people would have to
manually set format.noprefix=false. And it's unlikely that anybody
really wants format.noprefix=true in the first place. I'm adding it here
mostly as an escape hatch, not because anybody has expressed any
interest in it.

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