http: add support for specifying an SSL cipher list

Teach git about a new option, "http.sslCipherList", which permits one to
specify a list of ciphers to use when negotiating SSL connections.  The
setting can be overwridden by the GIT_SSL_CIPHER_LIST environment
variable.

Signed-off-by: Lars Kellogg-Stedman <lars@redhat.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1561,6 +1561,19 @@ http.savecookies::
 	If set, store cookies received during requests to the file specified by
 	http.cookiefile. Has no effect if http.cookiefile is unset.
 
+http.sslCipherList::
+  A list of SSL ciphers to use when negotiating an SSL connection.
+  The available ciphers depend on whether libcurl was built against
+  NSS or OpenSSL and the particular configuration of the crypto
+  library in use.  Internally this sets the 'CURLOPT_SSL_CIPHER_LIST'
+  option; see the libcurl documentation for more details on the format
+  of this list.
++
+Can be overridden by the 'GIT_SSL_CIPHER_LIST' environment variable.
+To force git to use libcurl's default cipher list and ignore any
+explicit http.sslCipherList option, set 'GIT_SSL_CIPHER_LIST' to the
+empty string.
+
 http.sslVerify::
 	Whether to verify the SSL certificate when fetching or pushing
 	over HTTPS. Can be overridden by the 'GIT_SSL_NO_VERIFY' environment

contrib/completion/git-completion.bash

@@ -2123,6 +2123,7 @@ _git_config ()
 		http.noEPSV
 		http.postBuffer
 		http.proxy
+		http.sslCipherList
 		http.sslCAInfo
 		http.sslCAPath
 		http.sslCert

http.c

@@ -35,6 +35,7 @@ char curl_errorstr[CURL_ERROR_SIZE];
 static int curl_ssl_verify = -1;
 static int curl_ssl_try;
 static const char *ssl_cert;
+static const char *ssl_cipherlist;
 #if LIBCURL_VERSION_NUM >= 0x070903
 static const char *ssl_key;
 #endif
@@ -153,6 +154,8 @@ static int http_options(const char *var, const char *value, void *cb)
 		curl_ssl_verify = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp("http.sslcipherlist", var))
+		return git_config_string(&ssl_cipherlist, var, value);
 	if (!strcmp("http.sslcert", var))
 		return git_config_string(&ssl_cert, var, value);
 #if LIBCURL_VERSION_NUM >= 0x070903
@@ -327,6 +330,13 @@ static CURL *get_curl_handle(void)
 	if (http_proactive_auth)
 		init_curl_http_auth(result);
 
+	if (getenv("GIT_SSL_CIPHER_LIST"))
+		ssl_cipherlist = getenv("GIT_SSL_CIPHER_LIST");
+
+	if (ssl_cipherlist != NULL && *ssl_cipherlist)
+		curl_easy_setopt(result, CURLOPT_SSL_CIPHER_LIST,
+				ssl_cipherlist);
+
 	if (ssl_cert != NULL)
 		curl_easy_setopt(result, CURLOPT_SSLCERT, ssl_cert);
 	if (has_cert_password())