Allow per-command pager config

Allow per-command pager config

There is great debate over whether some commands should set
up a pager automatically. This patch allows individuals to
set their own pager preferences for each command, overriding
the default. For example, to disable the pager for git
status:

  git config pager.status false

If "--pager" or "--no-pager" is specified on the command
line, it takes precedence over the config option.

There are two caveats:

  - you can turn on the pager for plumbing commands.
    Combined with "core.pager = always", this will probably
    break a lot of things. Don't do it.

  - This only works for builtin commands. The reason is
    somewhat complex:

    Calling git_config before we do setup_git_directory
    has bad side effects, because it wants to know where
    the git_dir is to find ".git/config". Unfortunately,
    we cannot call setup_git_directory indiscriminately,
    because some builtins (like "init") break if we do.

    For builtins, this is OK, since we can just wait until
    after we call setup_git_directory. But for aliases, we
    don't know until we expand (recursively) which command
    we're doing. This should not be a huge problem for
    aliases, which can simply use "--pager" or "--no-pager"
    in the alias as appropriate.

    For external commands, however, we don't know we even
    have an external command until we exec it, and by then
    it is too late to check the config.

    An alternative approach would be to have a config mode
    where we don't bother looking at .git/config, but only
    at the user and system config files. This would make the
    behavior consistent across builtins, aliases, and
    external commands, at the cost of not allowing per-repo
    pager config for at all.

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