Add :completions command.

common/content/commands.js

@@ -1447,6 +1447,15 @@ var Commands = Module("commands", {
             },
             { argCount: "0" });
 
+        commands.add(["comp[letions]"],
+            "List the completion results for a given command substring",
+            function (args) { completion.listCompleter("ex", args[0]); },
+            {
+                argCount: "1",
+                completer: function (context, args) completion.ex(context),
+                literal: 0
+            });
+
         commands.add(["delc[ommand]"],
             "Delete the specified user-defined command",
             function (args) {

common/locale/en-US/various.xml

@@ -14,6 +14,17 @@
 
 <h2 tag="various">Various commands</h2>
 
+<item>
+    <tags>:comp :completions</tags>
+    <spec>:comp<oa>letions</oa> <a>ex-command</a></spec>
+    <description>
+        <p>
+            List the completion results for a given command substring.
+        </p>
+        <example><ex>:completions :set! <str delim="">browser.tabs.</str></ex></example>
+    </description>
+</item>
+
 <item>
     <tags>:contexts</tags>
     <spec>:contexts <a>ex-command</a></spec>