mailinfo: fix in-body header continuations

mailinfo: fix in-body header continuations

An empty line should stop any pending in-body headers, and start the
actual body parsing.

This also modifies the original test for the in-body headers to actually
have a real commit body that starts with spaces, and changes the test to
check that the long line matches _exactly_, and doesn't get extra data
from the body.

Fixes:6b4b013f1884 ("mailinfo: handle in-body header continuations")
Cc: Jonathan Tan <jonathantanmy@google.com>
Cc: Jeff King <peff@peff.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>