Implement "Current" completion for bind.

When binding a key, the first row will be the current binding if the key
is already bound. This should make it easier for users to tell when they
are binding a key that is already bound, and what it is bound to.

qutebrowser/completion/models/miscmodels.py

@@ -120,14 +120,20 @@ def buffer():
     return model
 
 
-def bind(_key):
+def bind(key):
     """A CompletionModel filled with all bindable commands and descriptions.
 
     Args:
-        _key: the key being bound.
+        key: the key being bound.
     """
-    # TODO: offer a 'Current binding' completion based on the key.
     model = completionmodel.CompletionModel(column_widths=(20, 60, 20))
+    cmd_name = objreg.get('key-config').get_bindings_for('normal').get(key)
+
+    if cmd_name:
+        cmd = cmdutils.cmd_dict.get(cmd_name)
+        data = [(cmd_name, cmd.desc, key)]
+        model.add_category(listcategory.ListCategory("Current", data))
+
     cmdlist = _get_cmd_completions(include_hidden=True, include_aliases=True)
     model.add_category(listcategory.ListCategory("Commands", cmdlist))
     return model

tests/unit/completion/test_models.py

@@ -567,6 +567,9 @@ def test_bind_completion(qtmodeltester, monkeypatch, stubs, config_stub,
     qtmodeltester.check(model)
 
     _check_completions(model, {
+        "Current": [
+            ('stop', 'stop qutebrowser', 's'),
+        ],
         "Commands": [
             ('drop', 'drop all user data', ''),
             ('hide', '', ''),