pager: find out pager setting from configuration

pager: find out pager setting from configuration

It was very unfortunate that we added core.pager setting to the
configuration file; even when the underlying command does not care
if there is no git repository is involved (think "git diff --no-index"),
the user would now rightfully want the configuration setting to be
honored, which means we would need to read the configuration file before
we launch the pager.

This is a minimum change in the sense that it restores the old
behaviour of not even reading config in setup_git_directory(),
but have the core.pager honored when we know it matters.

Note that this does not cover "git -p --git-dir where command";
the -p option immediately trigger the pager settings before we
even see --git-dir to learn where the configuration file is, so
we will end up reading the configuration from the place where
we would _normally_ find the git repository.

Signed-off-by: Junio C Hamano <gitster@pobox.com>