scripts/send-pull-request: Avoid multiple chain headers

scripts/send-pull-request: Avoid multiple chain headers

When creating a patch set with cover letter using the
send-pull-request script, both the "In-Reply-To" and "References"
headers are appended twice in patch 2 and subsequent.

That's because git-format-patch already inserted them and then
git-send-email repeats that. Suppressing mail threading in
git-send-email with --no-thread avoids the problem and is the
right solution because it works regardless whether git-send-email is
called once or twicee.

Repeating these headers is a violation of RFC 2822 and can confuse
mail programs. For example, Patchwork does not detect a patch series
problem when there are these extra headers.

[YOCTO #10718]

(From OE-Core rev: 303a1aa3df43eb0b693d8602062fa33c4a08fdd6)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>