send-email: improve RFC2047 quote parsing

The RFC2047 unquoting, used to parse email addresses in From and Cc
headers, is broken in several ways:

* It erroneously substitutes ' ' for '_' in *the whole* header, even
  outside the quoted field. [Noticed by Christoph.]

* It is too liberal in its matching, and happily matches the start
  of one quoted chunk against the end of another, or even just
  something that looks like such an end. [Noticed by Junio.]

* It fundamentally cannot cope with encodings that are not a
  superset of ASCII, nor several (incompatible) encodings in the
  same header.

This patch fixes the first two by doing a more careful decoding of
the outer quoting (e.g. "=AB" to represent an octet whose value is
0xAB).  Fixing the fundamental issues is left for a future, more
intrusive, patch.

Noticed-by: Christoph Miebach <christoph.miebach@web.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9001-send-email.sh

@@ -841,6 +841,19 @@ test_expect_success $PREREQ '--compose adds MIME for utf8 subject' '
 	grep "^Subject: =?UTF-8?q?utf8-s=C3=BCbj=C3=ABct?=" msgtxt1
 '
 
+test_expect_success $PREREQ 'utf8 author is correctly passed on' '
+	clean_fake_sendmail &&
+	test_commit weird_author &&
+	test_when_finished "git reset --hard HEAD^" &&
+	git commit --amend --author "Füñný Nâmé <odd_?=mail@example.com>" &&
+	git format-patch --stdout -1 >funny_name.patch &&
+	git send-email --from="Example <nobody@example.com>" \
+	  --to=nobody@example.com \
+	  --smtp-server="$(pwd)/fake.sendmail" \
+	  funny_name.patch &&
+	grep "^From: Füñný Nâmé <odd_?=mail@example.com>" msgtxt1
+'
+
 test_expect_success $PREREQ 'detects ambiguous reference/file conflict' '
 	echo master > master &&
 	git add master &&