trace2: refactor setting process starting time

Create trace2_initialize_clock() and call from main() to capture
process start time in isolation and before other sub-systems are
ready.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/mingw.c

@@ -2569,6 +2569,8 @@ void mingw_startup(void)
 	wchar_t **wenv, **wargv;
 	_startupinfo si;
 
+	trace2_initialize_clock();
+
 	maybe_redirect_std_handles();
 
 	/* get wide char arguments and environment */