git am/mailinfo: Don't look at in-body headers when rebasing

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: Lukas Sandström <luksan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5100/sample.mbox

@@ -650,3 +650,36 @@ index b0b5d8f..461c47e 100644
  		convert_to_utf8(line, charset.buf);
 -- 
 1.6.4.1
+From nobody Mon Sep 17 00:00:00 2001
+From: A U Thor <a.u.thor@example.com>
+Subject: check bogus body header (from)
+Date: Fri, 9 Jun 2006 00:44:16 -0700
+
+From: bogosity
+  - a list
+  - of stuff
+---
+diff --git a/foo b/foo
+index e69de29..d95f3ad 100644
+--- a/foo
++++ b/foo
+@@ -0,0 +1 @@
++content
+
+From nobody Mon Sep 17 00:00:00 2001
+From: A U Thor <a.u.thor@example.com>
+Subject: check bogus body header (date)
+Date: Fri, 9 Jun 2006 00:44:16 -0700
+
+Date: bogus
+
+and some content
+
+---
+diff --git a/foo b/foo
+index e69de29..d95f3ad 100644
+--- a/foo
++++ b/foo
+@@ -0,0 +1 @@
++content
+