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.h

@@ -325,6 +325,7 @@ void trace2_thread_exit_fl(const char *file, int line);
 
 #define trace2_thread_exit() trace2_thread_exit_fl(__FILE__, __LINE__)
 
+struct key_value_info;
 /*
  * Emits a "def_param" message containing a key/value pair.
  *
@@ -334,7 +335,7 @@ void trace2_thread_exit_fl(const char *file, int line);
  * `core.abbrev`, `status.showUntrackedFiles`, or `--no-ahead-behind`.
  */
 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);
 
 #define trace2_def_param(param, value) \
 	trace2_def_param_fl(__FILE__, __LINE__, (param), (value))