Builtins: control the use of pager from the command table.

This moves the built-in "always-use-pager" logic for log family
to the command dispatch table of git wrapper.  This makes it
easier to change the default use of pager, and has an added
benefit that we fork and exec the pager early before packs are
mmapped.

Pointed out by Juergen Ruehle <j.ruehle@bmiag.de>.

Signed-off-by: Junio C Hamano <junkio@cox.net>