Handle linebreaks in custom completions. (#1162)

If a command/flag description contains
a linebreak then the shell completion script
will interpret this as new command/flag.

To fix this we only use the first line
from the description in the output.

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

custom_completions_test.go

@@ -561,7 +561,7 @@ func TestFlagNameCompletionInGoWithDesc(t *testing.T) {
 	}
 	rootCmd.AddCommand(childCmd)
 
-	rootCmd.Flags().IntP("first", "f", -1, "first flag")
+	rootCmd.Flags().IntP("first", "f", -1, "first flag\nlonger description for flag")
 	rootCmd.PersistentFlags().BoolP("second", "s", false, "second flag")
 	childCmd.Flags().String("subFlag", "", "sub flag")