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>
diff --git a/trace2.c b/trace2.c
index 0efc4e7..49c23bf 100644
--- a/trace2.c
+++ b/trace2.c
@@ -634,7 +634,7 @@ void trace2_thread_exit_fl(const char *file, int line)
 }
 
 void trace2_def_param_fl(const char *file, int line, const char *param,
-			 const char *value)
+			 const char *value, const struct key_value_info *kvi)
 {
 	struct tr2_tgt *tgt_j;
 	int j;
@@ -644,7 +644,7 @@ void trace2_def_param_fl(const char *file, int line, const char *param,
 
 	for_each_wanted_builtin (j, tgt_j)
 		if (tgt_j->pfn_param_fl)
-			tgt_j->pfn_param_fl(file, line, param, value);
+			tgt_j->pfn_param_fl(file, line, param, value, kvi);
 }
 
 void trace2_def_repo_fl(const char *file, int line, struct repository *repo)