mailinfo: handle in-body header continuations

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: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5100/patch0018--no-inbody-headers

@@ -0,0 +1,6 @@
+diff --git a/foo b/foo
+index e69de29..d95f3ad 100644
+--- a/foo
++++ b/foo
+@@ -0,0 +1 @@
++content