archive-tar: use parse_config_key when parsing config

This is fewer lines of code, but more importantly, fixes a
bogus pointer offset. We are looking for "tar." in the
section, but later assume that the dot we found is at offset
9, not 3. This is a holdover from an earlier iteration of
767cf45 which called the section "tarfilter".

As a result, we could erroneously reject some filters with
dots in their name, as well as read uninitialized memory.

Reported by (and test by) René Scharfe.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

archive-tar.c

@@ -327,20 +327,12 @@ static struct archiver *find_tar_filter(const char *name, int len)
 static int tar_filter_config(const char *var, const char *value, void *data)
 {
 	struct archiver *ar;
-	const char *dot;
 	const char *name;
 	const char *type;
 	int namelen;
 
-	if (prefixcmp(var, "tar."))
+	if (parse_config_key(var, "tar", &name, &namelen, &type) < 0 || !name)
 		return 0;
-	dot = strrchr(var, '.');
-	if (dot == var + 9)
-		return 0;
-
-	name = var + 4;
-	namelen = dot - name;
-	type = dot + 1;
 
 	ar = find_tar_filter(name, namelen);
 	if (!ar) {

t/t5000-tar-tree.sh

@@ -212,7 +212,8 @@ test_expect_success 'git-archive --prefix=olde-' '
 test_expect_success 'setup tar filters' '
 	git config tar.tar.foo.command "tr ab ba" &&
 	git config tar.bar.command "tr ab ba" &&
-	git config tar.bar.remote true
+	git config tar.bar.remote true &&
+	git config tar.invalid baz
 '
 
 test_expect_success 'archive --list mentions user filter' '