Documentation: teach "cmd-list.perl" about out-of-tree builds

Documentation: teach "cmd-list.perl" about out-of-tree builds

The "cmd-list.perl" script generates a list of commands that can be
included into our manpages. The script doesn't know about out-of-tree
builds and instead writes resulting files into the source directory.

Adapt it such that we can read data from the source directory and write
data into the build directory.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>