fsmonitor: preserve utf8 filenames in fsmonitor-watchman log

Update the test fsmonitor-watchman integration script to properly
preserve utf8 filenames when outputting the .git/watchman-output.out log
file.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7519/fsmonitor-watchman

@@ -129,6 +129,7 @@ sub launch_watchman {
 	    "Falling back to scanning...\n" if $o->{error};
 
 	open ($fh, ">", ".git/watchman-output.out");
+	binmode $fh, ":utf8";
 	print $fh @{$o->{files}};
 	close $fh;