Prevent tab completion if the word to complete is a single dash (#16781)

* added code to prevent tab completion if the word to complete is a single dash

* added a test for testing tab completion if the word to complete is a single dash

* Minor update and add comment

Co-authored-by: Dongbo Wang <dongbow@microsoft.com>

src/System.Management.Automation/engine/CommandCompletion/CompletionCompleters.cs

@@ -1329,7 +1329,8 @@ namespace System.Management.Automation
                         context.Options.Remove("LiteralPaths");
                 }
 
-                if (context.WordToComplete != string.Empty && context.WordToComplete.Contains('-'))
+                // The word to complete contains a dash and it's not the first character. We try command names in this case.
+                if (context.WordToComplete.IndexOf('-') > 0)
                 {
                     var commandResults = CompleteCommand(context);
                     if (commandResults != null)

test/powershell/Host/TabCompletion/TabCompletion.Tests.ps1

@@ -1435,6 +1435,13 @@ dir -Recurse `
             $res = TabExpansion2 -inputScript $inputStr -cursorColumn $inputStr.Length
             $res.CompletionMatches | Should -BeNullOrEmpty
         }
+
+        It "A single dash should not complete to anything" {
+            function test-{}
+            $inputStr = 'git -'
+            $res = TabExpansion2 -inputScript $inputStr -cursorColumn $inputStr.Length
+            $res.CompletionMatches | Should -BeNullOrEmpty
+        }
     }
 
     Context "Tab completion error tests" {