trace2: teach Git to log environment variables

Via trace2, Git can already log interesting config parameters (see the
trace2_cmd_list_config() function). However, this can grant an
incomplete picture because many config parameters also allow overrides
via environment variables.

To allow for more complete logs, we add a new trace2_cmd_list_env_vars()
function and supporting implementation, modeled after the pre-existing
config param logging implementation.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Acked-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace2.txt

@@ -656,7 +656,8 @@ The "exec_id" field is a command-unique id and is only useful if the
 ------------
 
 `"def_param"`::
-	This event is generated to log a global parameter.
+	This event is generated to log a global parameter, such as a config
+	setting, command-line flag, or environment variable.
 +
 ------------
 {