var doc: advertise current DEFAULT_PAGER and DEFAULT_EDITOR settings

var doc: advertise current DEFAULT_PAGER and DEFAULT_EDITOR settings

Document the default pager and editor chosen at compile time in the
git-var(1) manpage so users curious about what command _this_ copy of
git will fall back to when EDITOR, VISUAL, and PAGER are unset can
find the answer quickly.

In builds leaving those settings uncustomized, this patch makes the
manpage continue to say "usually vi" and "usually less" so the
formatted documentation is usable for a wide audience including users
of custom builds that change those settings.  If you would like your
copy of the docs to be less noncommittal, you will need to set
DEFAULT_PAGER=less and DEFAULT_EDITOR=vi explicitly.

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