send-email: align RFC 2047 decoding more closely with the spec
send-email: align RFC 2047 decoding more closely with the spec

More specifically:

* Add "\" to the list of characters not allowed in a token (see RFC 2047
  errata).

* Share regexes between unquote_rfc2047 and is_rfc2047_quoted. Besides
  removing duplication, this also makes unquote_rfc2047 more stringent.

* Allow both "q" and "Q" to identify the encoding.

* Allow lowercase hexadecimal digits in the "Q" encoding.

And, more on the cosmetic side:

* Change the "encoded-text" regex to exclude rather than include characters,
  for clarity and consistency with "token".

Signed-off-by: Роман Донченко <6410387d42ac69b5c3f5c2acf34f310f72f9d2c7@corrigendum.ru>
Acked-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git