check-docs: allow command-list.txt to contain excluded commands

check-docs: allow command-list.txt to contain excluded commands

Among other things, the `check-docs` target ensures that
`command-list.txt` no longer contains commands that were dropped (or
that were never added in the first place).

To do so, it compares the list of commands from that file to the
commands listed in `$(ALL_COMMANDS)`.

However, some build options exclude commands from the latter. Fix the
target to handle this situation correctly by taking the just-introduced
`$(EXCLUDED_PROGRAMS)` into account.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>