Allow HTTP user agent string to be modified.

Some firewalls restrict HTTP connections based on the clients user agent.  This
commit provides the user the ability to modify the user agent string via either
a new config option (http.useragent) or by an environment variable
(GIT_HTTP_USER_AGENT).

Relevant documentation is added to Documentation/config.txt.

Signed-off-by: Spencer E. Olson <olsonse@umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1243,6 +1243,15 @@ http.noEPSV::
 	support EPSV mode. Can be overridden by the 'GIT_CURL_FTP_NO_EPSV'
 	environment variable. Default is false (curl will use EPSV).
 
+http.useragent::
+	The HTTP USER_AGENT string presented to an HTTP server.  The default
+	value represents the version of the client git such as git/1.7.1.
+	This option allows you to override this value to a more common value
+	such as Mozilla/4.0.  This may be necessary, for instance, if
+	connecting through a firewall that restricts HTTP connections to a set
+	of common USER_AGENT strings (but not including those like git/1.7.1).
+	Can be overridden by the 'GIT_HTTP_USER_AGENT' environment variable.
+
 i18n.commitEncoding::
 	Character encoding the commit messages are stored in; git itself
 	does not care per se, but this information is necessary e.g. when

Makefile

@@ -1872,7 +1872,7 @@ builtin/init-db.s builtin/init-db.o: EXTRA_CPPFLAGS = \
 
 config.s config.o: EXTRA_CPPFLAGS = -DETC_GITCONFIG='"$(ETC_GITCONFIG_SQ)"'
 
-http.s http.o: EXTRA_CPPFLAGS = -DGIT_USER_AGENT='"git/$(GIT_VERSION)"'
+http.s http.o: EXTRA_CPPFLAGS = -DGIT_HTTP_USER_AGENT='"git/$(GIT_VERSION)"'
 
 ifdef NO_EXPAT
 http-walker.s http-walker.o: EXTRA_CPPFLAGS = -DNO_EXPAT

http.c

@@ -41,6 +41,7 @@ static long curl_low_speed_time = -1;
 static int curl_ftp_no_epsv;
 static const char *curl_http_proxy;
 static char *user_name, *user_pass;
+static const char *user_agent;
 
 #if LIBCURL_VERSION_NUM >= 0x071700
 /* Use CURLOPT_KEYPASSWD as is */
@@ -196,6 +197,9 @@ static int http_options(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (!strcmp("http.useragent", var))
+		return git_config_string(&user_agent, var, value);
+
 	/* Fall back on the default ones */
 	return git_default_config(var, value, cb);
 }
@@ -279,7 +283,8 @@ static CURL *get_curl_handle(void)
 	if (getenv("GIT_CURL_VERBOSE"))
 		curl_easy_setopt(result, CURLOPT_VERBOSE, 1);
 
-	curl_easy_setopt(result, CURLOPT_USERAGENT, GIT_USER_AGENT);
+	curl_easy_setopt(result, CURLOPT_USERAGENT,
+		user_agent ? user_agent : GIT_HTTP_USER_AGENT);
 
 	if (curl_ftp_no_epsv)
 		curl_easy_setopt(result, CURLOPT_FTP_USE_EPSV, 0);
@@ -380,6 +385,8 @@ void http_init(struct remote *remote)
 #endif
 	set_from_env(&ssl_cainfo, "GIT_SSL_CAINFO");
 
+	set_from_env(&user_agent, "GIT_HTTP_USER_AGENT");
+
 	low_speed_limit = getenv("GIT_HTTP_LOW_SPEED_LIMIT");
 	if (low_speed_limit != NULL)
 		curl_low_speed_limit = strtol(low_speed_limit, NULL, 10);