t6000lib: re-fix tr portability

It seems that some implementations of tr don't like a
replacement string of '-----...'; they try to find the
double-dash option "---...".

Instead of this pipeline of tr and sed invocations, just use a
single perl invocation.

Signed-off-by: Jeff King <peff@peff.net>

t/t6000lib.sh

@@ -97,10 +97,13 @@ check_output()
 # from front and back.
 name_from_description()
 {
-        tr "'" '-' |
-		tr '~`!@#$%^&*()_+={}[]|\;:"<>,/? ' \
-		   '------------------------------' |
-		tr -s '-' | tr '[A-Z]' '[a-z]' | sed "s/^-*//;s/-*\$//"
+	perl -pe '
+		s/[^A-Za-z0-9.]/-/g;
+		s/-+/-/g;
+		s/-$//;
+		s/^-//;
+		y/A-Z/a-z/;
+	'
 }