config: respect '~' and '~user' in mailmap.file

git_config_string() does not handle '~' and '~user' as part of the
value. Using git_config_pathname() fixes this.

Signed-off-by: Øystein Walle <oystwa@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -963,7 +963,7 @@ static int git_default_push_config(const char *var, const char *value)
 static int git_default_mailmap_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "mailmap.file"))
-		return git_config_string(&git_mailmap_file, var, value);
+		return git_config_pathname(&git_mailmap_file, var, value);
 	if (!strcmp(var, "mailmap.blob"))
 		return git_config_string(&git_mailmap_blob, var, value);