[settings-] prevent lockup when keystroke is same as longname

like from addCommand('a', 'a', 'pass', 'looping command')

Author: midichef

visidata/settings.py

@@ -383,8 +383,13 @@ def getCommand(sheet, cmd):
         return cmd
 
     longname = cmd
+    seen = []
     while vd.bindkeys._get(longname, obj=sheet) is not None:
         longname = vd.bindkeys._get(longname, obj=sheet)
+        if longname in seen:
+            vd.warning('keystroke/command definitions form a cycle')
+            break
+        seen.append(longname)
 
     return vd.commands._get(longname, obj=sheet)