format-patch: make format.noprefix a boolean

The config `format.noprefix` was added in 8d5213de (format-patch: add format.noprefix option, 2023-03-09) to support no-prefix on paths. That was immediately after making git-format-patch(1) not respect `diff.noprefix`.[1] The intent was to mirror `diff.noprefix`. But this config was unintentionally[2] implemented by enabling no-prefix if any kind of value is set. † 1: c169af8f (format-patch: do not respect diff.noprefix, 2023-03-09) † 2: https://lore.kernel.org/all/20260211073553.GA1867915@coredump.intra.peff.net/ Let’s indeed mirror `diff.noprefix` by treating it as a boolean. This is a breaking change. And as far as breaking changes go it is pretty benign: • The documentation claims that this config is equivalent to `diff.noprefix`; this is just a bug fix if the documentation is what defines the application interface • Only users with non-boolean values will run into problems when we try to parse it as a boolean. But what would (1) make them suspect they could do that in the first place, and (2) have motivated them to do it? • Users who have set this to `false` and expect that to mean *enable format.noprefix* (current behavior) will now have the opposite experience. Which is not a reasonable setup. Let’s only offer a breaking change fig leaf by advising about the previous behavior before dying. Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name> Signed-off-by: Junio C Hamano <gitster@pobox.com>