http: when using Secure Channel, ignore sslCAInfo by default

http: when using Secure Channel, ignore sslCAInfo by default

As of cURL v7.60.0, the Secure Channel backend can use the certificate
bundle provided via `http.sslCAInfo`, but that would override the
Windows Certificate Store. Since this is not desirable by default, let's
tell Git to not ask cURL to use that bundle by default when the `schannel`
backend was configured via `http.sslBackend`, unless
`http.schannelUseSSLCAInfo` overrides this behavior.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>