devel/magit: Update to 4.7.0

• Track upstream's replacement of docs/RelNotes/ with per-major-release docs/CHANGELOG.N files in PORTDOCS and the DOCS install
• Fix the DOCS and PDF options. With the custom do-build target, DOCS_ALL_TARGET/PDF_ALL_TARGET never ran, so the HTML and PDF manuals were never built and staging failed. Build them explicitly in post-build-DOCS-on/post-build-PDF-on, under MAKE_ENV so the sed=gsed binary alias is on PATH.

Changelog: https://github.com/magit/magit/blob/v4.7.0/docs/CHANGELOG.4#v470----2026-08-01
Sponsored by: The FreeBSD Foundation