check-docs: do not bother checking for legacy scripts' documentation

In the recent years, there has been a big push to convert more and more of Git's commands that are implemented as scripts to built-ins written in pure, portable C, for robustness, speed and portability. One strategy that served us well is to convert those scripts incrementally, starting by renaming the scripts to `git-legacy-<command>`, then introducing a built-in that does nothing else at first than checking the config setting `<command>.useBuiltin` (which defaults to `false` at the outset) and handing off to the legacy script if so asked. Obviously, those `git-legacy-<command>` commands share the documentation with the built-in `git-<command>`, and are not intended to be called directly anyway. So let's not try to ensure that they are documented separately from their built-in versions. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>