version: add git_user_agent function

This is basically a fancy way of saying "git/$GIT_VERSION",
except that it is overridable at build-time and through the
environment. Which means that people who don't want to
advertise their git version (for privacy or security
reasons) can tweak it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

version.c

@@ -2,3 +2,16 @@
 #include "version.h"
 
 const char git_version_string[] = GIT_VERSION;
+
+const char *git_user_agent(void)
+{
+	static const char *agent = NULL;
+
+	if (!agent) {
+		agent = getenv("GIT_USER_AGENT");
+		if (!agent)
+			agent = GIT_USER_AGENT;
+	}
+
+	return agent;
+}