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

@@ -4026,6 +4026,8 @@ int wmain(int argc, const wchar_t **wargv)
 
 	SetConsoleCtrlHandler(handle_ctrl_c, TRUE);
 
+	trace2_initialize_clock();
+
 	maybe_redirect_std_handles();
 	adjust_symlink_flags();
 	fsync_object_files = 1;