mailinfo: make ">From" in-body header check more robust

Since commit 81c5cf7 (mailinfo: skip bogus UNIX From line inside
body, 2006-05-21), we have treated lines like ">From" in the body as
headers. This makes "git am" work for people who erroneously paste
the whole output from format-patch:

  From 12345abcd...fedcba543210 Mon Sep 17 00:00:00 2001
  From: them
  Subject: [PATCH] whatever

into their email body (assuming that an mbox writer then quotes
"From" as ">From", as otherwise we would actually mailsplit on the
in-body line).

However, this has false positives if somebody actually has a commit
body that starts with "From "; in this case we erroneously remove
the line entirely from the commit message. We can make this check
more robust by making sure the line actually looks like a real mbox
"From" line.

Inspect the line that begins with ">From " a more carefully to only
skip lines that match the expected pattern (note that the datestamp
part of the format-patch output is designed to be kept constant to
help those who write magic(5) entries).

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

1 files changed, 10 insertions, 0 deletions

diff --git a/t/t5100/quoted-from.in b/t/t5100/quoted-from.in
new file mode 100644
index 0000000000..847e1c4d3e
--- /dev/null
+++ b/t/t5100/quoted-from.in
@@ -0,0 +1,10 @@
+From 1234567890123456789012345678901234567890 Mon Sep 17 00:00:00 2001
+From: Author Name <somebody@example.com>
+Date: Sun, 25 May 2008 00:38:18 -0700
+Subject: [PATCH] testing quoted >From
+
+>From the depths of history, we are stuck with the
+flaky mbox format.
+
+---
+patch