ed: allow repeated search with // and ??

* In commit 78d5429, repeat search was only implemented for "/" and "?"
* Add support for empty patterns specified as "??" and "//" (these may be followed by whitespace, but single "?" and "/" may not)
* test1: "/\n" --> repeat search down
* test2: "/ \n" --> start new search (down) for " "
* test3: "// \n" --> repeat search down
* test4: "?\n" --> repeat search up
* test5: "? \n"--> start new search (up) for " "
* test6: "?? \n --> repeat search up
* Entering //n doesn't seem to work for repeating search then issuing n command (this could be supported later)

bin/ed

@@ -886,14 +886,14 @@ sub edSetCurrentLine {
 
 sub edParse {
     s/\A\s+//;
-    if (m/\A(\/|\?)\z/) {
+    if (m/\A(\/|\?)\z/ || m/\A(\/{2}|\?{2})\s*\z/) {
         unless (defined $SearchPat) {
             edWarn(E_NOPAT);
             $command = 'nop';
             return 1;
         }
         my $found;
-        if ($1 eq '/') {
+        if ($1 eq '/' || $1 eq '//') {
             $found = edSearchForward($SearchPat);
         } else {
             $found = edSearchBackward($SearchPat);