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>

config.h

@@ -386,7 +386,6 @@ void git_global_config(char **user, char **xdg);
 
 int git_config_parse_parameter(const char *, config_fn_t fn, void *data);
 
-enum config_scope current_config_scope(void);
 const char *config_origin_type_name(enum config_origin_type type);
 void kvi_from_param(struct key_value_info *out);