docs: exclude documentation for commands that have been excluded

When building with certain build options, some commands are excluded from the build. For example, `git-credential-cache` is skipped when building with `NO_UNIX_SOCKETS`. Let's not build or package documentation for those excluded commands. This issue was pointed out rightfully when running `make check-docs` on Windows, where we do not yet have Unix sockets, and therefore the `credential-cache` command is excluded (yet its documentation was built and shipped). Note: building the documentation via `make -C Documentation` leaves the build system with no way to determine which commands have been excluded. If called thusly, we gracefully fail to exclude their documentation. Only when building the documentation via the top-level Makefile will it get excluded properly, or after building `Documentation/GIT-EXCLUDED-PROGRAMS` manually. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>