credential: use the last matching username in the config

Everywhere else in the codebase, we use the rule that the last matching
configuration option is the one that takes effect.  This is helpful
because it allows more specific configuration settings (e.g., per-repo
configuration) to override less specific settings (e.g., per-user
configuration).

However, in the credential code, we didn't honor this setting, and
instead picked the first setting we had, and stuck with it.  This was
likely to ensure we picked the value from the URL, which we want to
honor over the configuration.

It's possible to do both, though, so let's check if the value is the one
we've gotten over our protocol connection, which if present will have
come from the URL, and keep it if so.  Otherwise, let's overwrite the
value with the latest version we've got from the configuration, so we
keep the last configuration value.

Signed-off-by: brian m. carlson <bk2204@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

credential.h

@@ -208,7 +208,8 @@ struct credential {
 	unsigned approved:1,
 		 configured:1,
 		 quit:1,
-		 use_http_path:1;
+		 use_http_path:1,
+		 username_from_proto:1;
 
 	char *username;
 	char *password;