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>

Documentation/config.txt

diff --git a/Documentation/config.txt b/Documentation/config.txt
index e107f4c..56a1104 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2249,6 +2249,14 @@
 	certificate. This option is ignored if cURL lacks support for
 	setting the relevant SSL option at runtime.
 
+http.schannelUseSSLCAInfo::
+	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, Git will tell cURL not to use that bundle by default
+	when the `schannel` backend was configured via `http.sslBackend`,
+	unless `http.schannelUseSSLCAInfo` overrides this behavior.
+
 http.pinnedpubkey::
 	Public key of the https service. It may either be the filename of
 	a PEM or DER encoded public key file or a string starting with

http.c

diff --git a/http.c b/http.c
index 272584b..43e75ac 100644
--- a/http.c
+++ b/http.c
@@ -158,6 +158,12 @@ static char *cached_accept_language;
 static char *http_ssl_backend;
 
 static int http_schannel_check_revoke = 1;
+/*
+ * With the backend being set to `schannel`, setting sslCAinfo would override
+ * the Certificate Store in cURL v7.60.0 and later, which is not what we want
+ * by default.
+ */
+static int http_schannel_use_ssl_cainfo;
 
 size_t fread_buffer(char *ptr, size_t eltsize, size_t nmemb, void *buffer_)
 {
@@ -317,6 +323,11 @@ static int http_options(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp("http.schannelusesslcainfo", var)) {
+		http_schannel_use_ssl_cainfo = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp("http.minsessions", var)) {
 		min_curl_sessions = git_config_int(var, value);
 #ifndef USE_CURL_MULTI
@@ -869,7 +880,13 @@ static CURL *get_curl_handle(void)
 	if (ssl_pinnedkey != NULL)
 		curl_easy_setopt(result, CURLOPT_PINNEDPUBLICKEY, ssl_pinnedkey);
 #endif
-	if (ssl_cainfo != NULL)
+	if (http_ssl_backend && !strcmp("schannel", http_ssl_backend) &&
+	    !http_schannel_use_ssl_cainfo) {
+		curl_easy_setopt(result, CURLOPT_CAINFO, NULL);
+#if LIBCURL_VERSION_NUM >= 0x073400
+		curl_easy_setopt(result, CURLOPT_PROXY_CAINFO, NULL);
+#endif
+	} else if (ssl_cainfo != NULL)
 		curl_easy_setopt(result, CURLOPT_CAINFO, ssl_cainfo);
 
 	if (curl_low_speed_limit > 0 && curl_low_speed_time > 0) {