trace2: add a public function for getting the SID

Add a public wrapper, trace2_session_id(), around tr2_sid_get(), which
is intended to be private trace2 implementation.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

trace2.c

@@ -792,3 +792,8 @@ void trace2_printf(const char *fmt, ...)
 	va_end(ap);
 }
 #endif
+
+const char *trace2_session_id(void)
+{
+	return tr2_sid_get();
+}

trace2.h

@@ -500,4 +500,6 @@ void trace2_collect_process_info(enum trace2_process_info_reason reason);
 	} while (0)
 #endif
 
+const char *trace2_session_id(void);
+
 #endif /* TRACE2_H */