Add --source flag back to hugo new command

v0.15 had it, but I removed it inadvertently for 0.16-DEV
in commit 00d0477 in PR gohugoio#1652.

Also add directory bash-completion for `--source`
for `hugo convert`, `hugo list` and `hugo new`.

See gohugoio#1624, gohugoio#1589

commands/convert.go

@@ -76,6 +76,7 @@ func init() {
 	convertCmd.PersistentFlags().StringVarP(&outputDir, "output", "o", "", "filesystem path to write files to")
 	convertCmd.PersistentFlags().StringVarP(&Source, "source", "s", "", "filesystem path to read files relative from")
 	convertCmd.PersistentFlags().BoolVar(&unsafe, "unsafe", false, "enable less safe operations, please backup first")
+	convertCmd.PersistentFlags().SetAnnotation("source", cobra.BashCompSubdirsInDir, []string{})
 }
 
 func convertContents(mark rune) (err error) {

commands/list.go

@@ -26,6 +26,7 @@ func init() {
 	listCmd.AddCommand(listDraftsCmd)
 	listCmd.AddCommand(listFutureCmd)
 	listCmd.PersistentFlags().StringVarP(&Source, "source", "s", "", "filesystem path to read files relative from")
+	listCmd.PersistentFlags().SetAnnotation("source", cobra.BashCompSubdirsInDir, []string{})
 }
 
 var listCmd = &cobra.Command{

commands/new.go

@@ -41,6 +41,8 @@ func init() {
 	newSiteCmd.Flags().Bool("force", false, "Init inside non-empty directory")
 	newCmd.Flags().StringVarP(&configFormat, "format", "f", "toml", "frontmatter format")
 	newCmd.Flags().StringVarP(&contentType, "kind", "k", "", "Content type to create")
+	newCmd.PersistentFlags().StringVarP(&Source, "source", "s", "", "filesystem path to read files relative from")
+	newCmd.PersistentFlags().SetAnnotation("source", cobra.BashCompSubdirsInDir, []string{})
 	newCmd.AddCommand(newSiteCmd)
 	newCmd.AddCommand(newThemeCmd)
 }