http: add environment variable support for HTTPS proxies

Add 4 environment variables that can be used to configure the proxy
cert, proxy ssl key, the proxy cert password protected flag, and the
CA info for the proxy.

Documentation for the options was also updated.

Signed-off-by: Jorge Lopez Silva <jalopezsilva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/http.txt

@@ -31,20 +31,24 @@ http.proxyAuthMethod::
 
 http.proxySSLCert::
 	The pathname of a file that stores a client certificate to use to authenticate
-	with an HTTPS proxy.
+	with an HTTPS proxy. Can be overridden by the `GIT_PROXY_SSL_CERT` environment
+	variable.
 
 http.proxySSLKey::
 	The pathname of a file that stores a private key to use to authenticate with
-	an HTTPS proxy.
+	an HTTPS proxy. Can be overridden by the `GIT_PROXY_SSL_KEY` environment
+	variable.
 
 http.proxySSLCertPasswordProtected::
 	Enable Git's password prompt for the proxy SSL certificate.  Otherwise OpenSSL
 	will prompt the user, possibly many times, if the certificate or private key
-	is encrypted.
+	is encrypted. Can be overriden by the `GIT_PROXY_SSL_CERT_PASSWORD_PROTECTED`
+	environment variable.
 
 http.proxySSLCAInfo::
 	Pathname to the file containing the certificate bundle that should be used to
-	verify the proxy with when using an HTTPS proxy.
+	verify the proxy with when using an HTTPS proxy. Can be overriden by the
+	`GIT_PROXY_SSL_CAINFO` environment variable.
 
 http.emptyAuth::
 	Attempt authentication without seeking a username or password.  This

http.c

@@ -1211,6 +1211,13 @@ void http_init(struct remote *remote, const char *url, int proactive_auth)
 		max_requests = DEFAULT_MAX_REQUESTS;
 #endif
 
+	set_from_env(&http_proxy_ssl_cert, "GIT_PROXY_SSL_CERT");
+	set_from_env(&http_proxy_ssl_key, "GIT_PROXY_SSL_KEY");
+	set_from_env(&http_proxy_ssl_ca_info, "GIT_PROXY_SSL_CAINFO");
+
+	if (getenv("GIT_PROXY_SSL_CERT_PASSWORD_PROTECTED"))
+		proxy_ssl_cert_password_required = 1;
+
 	if (getenv("GIT_CURL_FTP_NO_EPSV"))
 		curl_ftp_no_epsv = 1;