tests: Add tests for automatic use of pager

Git’s automatic pagination support has some subtleties.  Add some
tests to make sure we don’t break:

 - when git will use a pager by default;
 - the effect of the --paginate and --no-pager options;
 - the effect of pagination on use of color;
 - how the choice of pager is configured.

This does not yet test:

 - use of pager by scripted commands (git svn and git am);
 - effect of the pager.* configuration variables;
 - setting of the LESS variable.

Some features involve checking whether stdout is a terminal, so many
of these tests are skipped unless output is passed through to the
terminal (i.e., unless $GIT_TEST_OPTS includes --verbose).

The immediate purpose for these tests was to avoid making things worse
after the breakage from my jn/editor-pager series (see commit 376f39,
2009-11-20).  Thanks to Sebastian Celis <sebastian@sebastiancelis.com>
for the report.

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

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