http: add support for specifying the SSL version

Teach git about a new option, "http.sslVersion", which permits one
to specify the SSL version to use when negotiating SSL connections.
The setting can be overridden by the GIT_SSL_VERSION environment
variable.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2118,6 +2118,7 @@ _git_config ()
 		http.postBuffer
 		http.proxy
 		http.sslCipherList
+		http.sslVersion
 		http.sslCAInfo
 		http.sslCAPath
 		http.sslCert