git.c: provide setup_auto_pager()

git.c: provide setup_auto_pager()

The previous patch introduced a way for builtins to declare that they
will take responsibility for handling the `pager.foo`-config item. (See
the commit message of that patch for why that could be useful.)

Provide setup_auto_pager(), which builtins can call in order to handle
`pager.<cmd>`, including possibly starting the pager. Make this function
don't do anything if a pager has already been started, as indicated by
use_pager or pager_in_use().

Whenever this function is called from a builtin, git.c will already have
called commit_pager_choice(). Since commit_pager_choice() treats the
special value -1 as "punt" or "not yet decided", it is not a problem
that we might end up calling commit_pager_choice() once in git.c and
once (or more) in the builtin. Make the new function use -1 in the same
way and document it as "punt".

Don't add any users of setup_auto_pager just yet, one will follow in
a later patch.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>