t: test subject handling in format-patch / am pipeline

t: test subject handling in format-patch / am pipeline

Commit a1f6baa (format-patch: wrap long header lines,
2011-02-23) changed format-patch's behavior with respect to
long header lines, but made no accompanying changes to the
receiving side. It was thought that "git am" would handle
these folded subjects fine, but there is a regression when
using "am -k".

Let's add a test documenting this. While we're at it, let's
give more complete test coverage to document what should be
happening in each case. We test three types of subjects:
a short one, one long enough to require wrapping, and a
multiline subject. For each, we test these three
combinations:

  format-patch | am
  format-patch -k | am
  format-patch -k | am -k

We don't bother testing "format-patch | am -k", which is
nonsense (you will be adding in [PATCH] cruft to each
subject).

This reveals the regression above (long subjects have
linebreaks introduced via "format-patch -k | am -k"),
as well as an existing non-optimal behavior (multiline
subjects are not preserved using "-k").

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>