builtin-help: silently tolerate unknown keys

builtin-help: silently tolerate unknown keys

If for some reason the config file contains a key without a subkey like

    [man]
        foo = bar

then even a plain

    git help

produces an error message. With this patch such an entry is ignored.

Additionally, the warning about unknown sub-keys is removed. It could
become annoying if new sub-keys are introduced in the future, and then
the configuration is read by an old version of git that does not know
about it.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>