mailinfo: do not concatenate charset= attribute values from mime headers

"Content-type: text/plain; charset=UTF-8" header should not appear
twice in the input, but it is always better to gracefully deal with
such a case.  The current code concatenates the value to the values
we have seen previously, producing nonsense such as "utf8UTF-8".

Instead of concatenating, forget the previous value and use the last
value we see.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/mailinfo.c

@@ -160,10 +160,9 @@ static int slurp_attr(const char *line, const char *name, struct strbuf *attr)
 	const char *ends, *ap = strcasestr(line, name);
 	size_t sz;
 
-	if (!ap) {
+	strbuf_setlen(attr, 0);
-		strbuf_setlen(attr, 0);
+	if (!ap)
 		return 0;
-	}
 	ap += strlen(name);
 	if (*ap == '"') {
 		ap++;

t/t5100-mailinfo.sh

@@ -11,7 +11,7 @@ test_expect_success 'split sample box' \
 	'git mailsplit -o. "$TEST_DIRECTORY"/t5100/sample.mbox >last &&
 	last=`cat last` &&
 	echo total is $last &&
-	test `cat last` = 16'
+	test `cat last` = 17'
 
 check_mailinfo () {
 	mail=$1 opt=$2

t/t5100/info0017

@@ -0,0 +1,5 @@
+Author: A U Thor
+Email: a.u.thor@example.com
+Subject: A E I O U
+Date: Mon, 17 Sep 2012 14:23:44 -0700
+

t/t5100/msg0017

@@ -0,0 +1,2 @@
+New content here
+

t/t5100/patch0017

@@ -0,0 +1,6 @@
+diff --git a/foo b/foo
+index e69de29..d95f3ad 100644
+--- a/foo
++++ b/foo
+@@ -0,0 +1 @@
++New content

t/t5100/sample.mbox

@@ -683,3 +683,19 @@ index e69de29..d95f3ad 100644
 @@ -0,0 +1 @@
 +content
 
+From nobody Mon Sep 17 00:00:00 2001
+From: A U Thor <a.u.thor@example.com>
+Subject: A E I O U
+Date: Mon, 17 Sep 2012 14:23:44 -0700
+MIME-Version: 1.0
+Content-Type: text/plain; charset="iso-2022-jp"
+Content-type: text/plain; charset="UTF-8"
+
+New content here
+
+diff --git a/foo b/foo
+index e69de29..d95f3ad 100644
+--- a/foo
++++ b/foo
+@@ -0,0 +1 @@
++New content