git --paginate: do not commit pager choice too early

When git is passed the --paginate option, starting up a pager requires
deciding what pager to start, which requires access to the core.pager
configuration.

At the relevant moment, the repository has not been searched for yet.
Attempting to access the configuration at this point results in
git_dir being set to .git [*], which is almost certainly not what was
wanted.  In particular, when run from a subdirectory of the toplevel,
git --paginate does not respect the core.pager setting from the
current repository.

[*] unless GIT_DIR or GIT_CONFIG is set

So delay the pager startup when possible:

1. run_argv() already commits pager choice inside run_builtin() if a
   command is found.  For commands that use RUN_SETUP, waiting until
   then fixes the problem described above: once git knows where to
   look, it happily respects the core.pager setting.

2. list_common_cmds_help() prints out 29 lines and exits.  This can
   benefit from pagination, so we need to commit the pager choice
   before writing this output.

   Luckily ‘git’ without subcommand has no other reason to access a
   repository, so it would be intuitive to ignore repository-local
   configuration in this case.  Simpler for now to choose a pager
   using the funny code that notices a repository that happens to be
   at .git.  That this accesses a repository when it is very
   convenient to is a bug but not an important one.

3. help_unknown_cmd() prints out a few lines to stderr.  It is not
   important to paginate this, so don’t.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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