Update repository.go to add a comment for the public API and fix Runtime issue.

repository.go

@@ -165,12 +165,17 @@ func (v *Repository) Config() (*Config, error) {
 	return config, nil
 }
 
+// SetConfig sets the configuration file for this repository.
+//
+// This configuration file will be used for all configuration queries involving
+// this repository.
 func (v *Repository) SetConfig(c *Config) error {
 	runtime.LockOSThread()
 	defer runtime.UnlockOSThread()
 
 	ret := C.git_repository_set_config(v.ptr, c.ptr)
 	runtime.KeepAlive(v)
+	runtime.KeepAlive(c)
 	if ret < 0 {
 		return MakeGitError(ret)
 	}