t7006 (pager): introduce helper for parameterized tests

t7006 (pager): introduce helper for parameterized tests

The current tests test pager configuration for ‘git log’, but other
commands use a different setup procedure and should therefore be
tested separately.  Add a helper to make this easier.

This patch introduces the helper and changes some existing tests to
use it.  The only functional change should be the introduction of ‘git
log - ’ to a few test descriptions.

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