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>