config: add support for http.<url>.* settings

The <url> value is considered a match to a url if the <url> value
is either an exact match or a prefix of the url which ends on a
path component boundary ('/').  So "https://example.com/test" will
match "https://example.com/test" and "https://example.com/test/too"
but not "https://example.com/testextra".

Longer matches take precedence over shorter matches with
environment variable settings taking precedence over all.

With this configuration:

[http]
        useragent = other-agent
        noEPSV = true
[http "https://example.com"]
        useragent = example-agent
        sslVerify = false
[http "https://example.com/path"]
        useragent = path-agent

The "https://other.example.com/" url will have useragent
"other-agent" and sslVerify will be on.

The "https://example.com/" url will have useragent
"example-agent" and sslVerify will be off.

The "https://example.com/path/sub" url will have useragent
"path-agent" and sslVerify will be off.

All three of the examples will have noEPSV enabled.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'km/http-curl-config-per-url' into pu

* km/http-curl-config-per-url:
  config: add support for http.<url>.* settings