add receive.denyNonFastforwards config variable

If receive.denyNonFastforwards is set to true, git-receive-pack will deny
non fast-forwards, i.e. forced updates. Most notably, a push to a repository
which has that flag set will fail.

As a first user, 'git-init-db --shared' sets this flag, since in a shared
setup, you are most unlikely to want forced pushes to succeed.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-init-db.c

@@ -311,6 +311,7 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 		 */
 		sprintf(buf, "%d", shared_repository);
 		git_config_set("core.sharedrepository", buf);
+		git_config_set("receive.denyNonFastforwards", "true");
 	}
 
 	return 0;