git svn: Fix launching of pager

git svn: Fix launching of pager

In commit dec543e (am -i, git-svn: use "git var GIT_PAGER"), I tried
to teach git svn to defer to git var on what pager to use. In the
process, I introduced two bugs:

 - The value set for $pager in config_pager has local scope, so
   run_pager never sees it;

 - git var cannot tell whether git svn’s output is going to a
   terminal, so the value chosen for $pager does not reflect that
   information.

Fix them.

Reported-by: Sebastian Celis <sebastian@sebastiancelis.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>