/join now has an option to complete existing chans

commands.join-tabexistingchannels will toggle tab completion for channels that
the user is already in. Defaults to false as most users will just use /join
for joining, not for trying to focus existing windows.

Fixes issue 4092

Change-Id: Ie5c260b202ff8617a28edb90838e4b6a57b7f326
Reviewed-on: http://gerrit.dmdirc.com/1201
Automatic-Compile: DMDirc Local Commits <dmdirc@googlemail.com>
Reviewed-by: Gregory Holmes <greg@dmdirc.com>

src/com/dmdirc/commandparser/commands/server/JoinChannelCommand.java

@@ -144,10 +144,15 @@ public final class JoinChannelCommand extends ServerCommand implements
             prefix = "";
         }
 
-        for (String result : results) {
-            // Only tab complete channels we're not already on
-            if (!server.hasChannel(result)) {
-                targets.add(prefix + result);
+        final boolean showExisting = source.getConfigManager()
+                .getOptionBool("commands", "join-tabexistingchannels");
+
+        if (!showExisting) {
+            for (String result : results) {
+                // Only tab complete channels we're not already on
+                if (!server.hasChannel(result)) {
+                    targets.add(prefix + result);
+                }
             }
         }