t/test-lib: fix TAP format for BASH_XTRACEFD warning

When the Bash version is too old to support BASH_XTRACEFD we print a
warning to stderr. This warning is not prefixed with "#", which causes
TAP parsers to (wrongly) interpret the warning as part of the protocol.

Fix this issue by prefixing the warning with a "#" so that it is treated
as comment.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

t/test-lib.sh

@@ -470,7 +470,7 @@ then
 	then
 		: Executed by a Bash version supporting BASH_XTRACEFD.  Good.
 	else
-		echo >&2 "warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
+		echo >&2 "# warning: ignoring -x; '$0' is untraceable without BASH_XTRACEFD"
 		trace=
 	fi
 fi