config: add generic callback wrapper to parse section.<url>.key

config: add generic callback wrapper to parse section.<url>.key

Existing configuration parsing functions (e.g. http_options() in
http.c) know how to parse two-level configuration variable names.
We would like to exploit them and parse something like this:

	[http]
		sslVerify = true
	[http "https://weak.example.com"]
		sslVerify = false

and pretend as if http.sslVerify were set to false when talking to
"https://weak.example.com/path".

Introduce `urlmatch_config_entry()` wrapper that:

 - is called with the target URL (e.g. "https://weak.example.com/path"),
   and the two-level variable parser (e.g. `http_options`);

 - uses `url_normalize()` and `match_urls()` to see if configuration
   data matches the target URL; and

 - calls the traditional two-level configuration variable parser
   only for the configuration data whose <url> part matches the
   target URL (and if there are multiple matches, only do so if the
   current match is a better match than the ones previously seen).

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