Documentation/Makefile: fix lint-docs mkdir dependency

Documentation/Makefile: fix lint-docs mkdir dependency

Since 8650c6298c (doc lint: make "lint-docs" non-.PHONY, 2021-10-15), we
put the output for gitlink linter into .build/lint-docs/gitlink. There
are order-only dependencies to create the sequence of subdirs like:

  .build/lint-docs: | .build
          $(QUIET)mkdir $@
  .build/lint-docs/gitlink: | .build/lint-docs
          $(QUIET)mkdir $@

where each level has to depend on the prior one (since the parent
directory must exist for us to create something inside it). But the
"howto" and "config" subdirectories of gitlink have the wrong
dependency; they depend on "lint-docs", not "lint-docs/gitlink".

This usually works out, because the LINT_DOCS_GITLINK targets which
depend on "gitlink/howto" also depend on just "gitlink", so the
directory gets created anyway. But since we haven't given make an
explicit ordering, things can racily happen out of order.

If you stick a "sleep 1" in the rule to build "gitlink" like this:

   ## Lint: gitlink
   .build/lint-docs/gitlink: | .build/lint-docs
  -	$(QUIET)mkdir $@
  +	$(QUIET)sleep 1 && mkdir $@

then "make clean; make lint-docs" will fail reliably. Or you can see it
as-is just by building the directory in isolation:

  $ make clean
  [...]
  $ make .build/lint-docs/gitlink/howto
      GEN mergetools-list.made
      GEN cmd-list.made
      GEN doc.dep
      SUBDIR ../
  make[1]: 'GIT-VERSION-FILE' is up to date.
      SUBDIR ../
  make[1]: 'GIT-VERSION-FILE' is up to date.
  mkdir: cannot create directory ‘.build/lint-docs/gitlink/howto’: No such file or directory
  make: *** [Makefile:476: .build/lint-docs/gitlink/howto] Error 1

The fix is easy: we just need to depend on the correct parent directory.

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