docs: exclude documentation for commands that have been excluded

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>