allow command-specific pagers in pager.<cmd>

allow command-specific pagers in pager.<cmd>

A user may want different pager settings or even a
different pager for various subcommands (e.g., because they
use different less settings for "log" vs "diff", or because
they have a pager that interprets only log output but not
other commands).

This patch extends the pager.<cmd> syntax to support not
only boolean to-page-or-not-to-page, but also to specify a
pager just for a specific command.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>