Add user preference "HistoryBackwardSearchUntilChanged"

When executing the same command multiple times, it is stored in history
multiple times and also appears multiple times when doing a backward search
in the history. Setting this option to <K>true</K> ensures that during a
backward search a history item is selected which actually changes the
currently displayed line. Note: This does not affect forward searches.

lib/cmdledit.g

@@ -64,6 +64,20 @@ readline support.",
   default := "default",
   ) );
 
+DeclareUserPreference( rec(
+  name:= "HistoryBackwardSearchUntilChanged",
+  description:= [
+    "When executing the same command multiple times, it is stored in history \
+multiple times and also appears multiple times when doing a backward search \
+in the history. Setting this option to <K>true</K> ensures that during a \
+backward search a history item is selected which actually changes the \
+currently displayed line. Note: This does not affect forward searches."
+    ],
+  default:= false,
+  values:= [ true, false ],
+  multi:= false,
+  ) );
+
 
 if GAPInfo.CommandLineOptions.E then
 ############################################################################
@@ -543,6 +557,9 @@ GAPInfo.CommandLineEditFunctions.Functions.BackwardHistory := function(l)
   while n > 1 do
     n := n - 1;
     if PositionSublist(hist[n], start) = 1 then
+      if UserPreference("HistoryBackwardSearchUntilChanged") and hist[n] = l[3] then
+        continue;
+      fi;
       GAPInfo.History.Pos := n;
       GAPInfo.History.Last := n;
       return [1, Length(l[3])+1, hist[n], l[4]];