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>

Makefile

@@ -3074,7 +3074,7 @@ check-docs::
 		git-merge-octopus | git-merge-ours | git-merge-recursive | \
 		git-merge-resolve | git-merge-subtree | \
 		git-fsck-objects | git-init-db | \
-		git-remote-* | git-stage | \
+		git-remote-* | git-stage | git-legacy-* | \
 		git-?*--?* ) continue ;; \
 		esac ; \
 		test -f "Documentation/$$v.txt" || \