trace2: plumb config kvi

There is a code path starting from trace2_def_param_fl() that eventually
calls current_config_scope(), and thus it needs to have "kvi" plumbed
through it. Additional plumbing is also needed to get "kvi" to
trace2_def_param_fl(), which gets called by two code paths:

- Through tr2_cfg_cb(), which is a config callback, so it trivially
  receives "kvi" via the "struct config_context ctx" parameter.

- Through tr2_list_env_vars_fl(), which is a high level function that
  lists environment variables for tracing. This has been secretly
  behaving like git_config_from_parameters() (in that it parses config
  from environment variables/the CLI), but does not set config source
  information.

  Teach tr2_list_env_vars_fl() to be well-behaved by using
  kvi_from_param(), which is used elsewhere for CLI/environment
  variable-based config.

As a result, current_config_scope() has no more callers, so remove it.

Signed-off-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2/tr2_tgt_perf.c

@@ -439,12 +439,12 @@ static void fn_exec_result_fl(const char *file, int line,
 }
 
 static void fn_param_fl(const char *file, int line, const char *param,
-			const char *value)
+			const char *value, const struct key_value_info *kvi)
 {
 	const char *event_name = "def_param";
 	struct strbuf buf_payload = STRBUF_INIT;
 	struct strbuf scope_payload = STRBUF_INIT;
-	enum config_scope scope = current_config_scope();
+	enum config_scope scope = kvi->scope;
 	const char *scope_name = config_scope_name(scope);
 
 	strbuf_addf(&buf_payload, "%s:%s", param, value);