Mailinfo currently handles multi-line headers, but it does not handle
multi-line in-body headers. Teach it to handle such headers, for
example, for this input:

  From: author <author@example.com>
  Date: Fri, 9 Jun 2006 00:44:16 -0700
  Subject: a very long
   broken line

  Subject: another very long
   broken line

interpret the in-body subject to be "another very long broken line"
instead of "another very long".

An existing test (t/t5100/msg0015) has an indented line immediately
after an in-body header - it has been modified to reflect the new
functionality.
Signed-off-by: NJonathan Tan <jonathantanmy@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

When we are rebasing we know that the header lines in the
patch are good and that we don't need to pick up any headers
from the body of the patch.

This makes it possible to rebase commits whose commit message
start with "From" or "Date".

Test vectors by Jeff King.
Signed-off-by: NLukas Sandström <luksan@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>