meson: consistently use custom program paths to resolve programs

meson: consistently use custom program paths to resolve programs

The calls to `find_program()` in our documentation don't use our custom
program path. This variable gets populated on Windows with the location
of Git for Windows so that we can use it to provide our build tools.
Consequently, we may not be able to find all necessary binaries on
Windows.

Adapt the calls to use the program path to fix this. While at it, drop
`required: true` arguments, which are the default anyway.

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