format-patch: make cover letters always text/plain

format-patch: make cover letters always text/plain

When formatting a series of patches using --attach and --cover-letter,
the cover letter lacks the closing MIME boundary, violating RFC 2046.
Certain clients, such as Thunderbird, discard the message body in such a
case.

Since the cover letter is just one part and sending it as
multipart/mixed is not very useful, always emit it as text/plain,
avoiding the boundary problem altogether.

Reported-by: Patrick Hemmer <git@stormcloud9.net>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>