git --paginate: paginate external commands again

git --paginate: paginate external commands again

73e25e7c (git --paginate: do not commit pager choice too early,
2010-06-26) failed to take some cases into account.

1b. Builtins that do not use RUN_SETUP (like git config) do
    not find GIT_DIR set correctly when the pager is launched
    from run_builtin().  So the core.pager configuration is
    not honored from subdirectories of the toplevel for them.

4a. External git commands (like git request-pull) relied on the
    early pager launch to take care of handling the -p option.
    Ever since 73e25e7c, they do not honor the -p option at all.

4b. Commands invoked through ! aliases (like ls) were also relying
    on the early pager launch.

Fix (4a) by launching the pager (if requested) before running such a
“dashed external”.  For simplicity, this still does not search for a
.git directory before running the external command; when run from a
subdirectory of the toplevel, therefore, the “[core] pager”
configuration is still not honored.

Fix (4b) by launching pager if requested before carrying out such an
alias.  Actually doing this has no effect, since the pager (if any)
would have already been launched in a failed attempt to try a
dashed external first.  The choice-of-pager-not-honored-from-
subdirectory bug still applies here, too.

(1b) is not a regression.  There is no need to fix it yet.

Noticed by Junio.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>