refactor(completion): name path arguments explicitly and share firstArg

The six commands that take a file or directory argument set their
completer directly instead of a Use-line parser wrapping every
command, the first-argument-only wrappers become one firstArg helper,
and only the bash completion help keeps a custom text.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/comments.go

@@ -426,13 +426,7 @@ func article(noun string) string {
 // completeDiscussions completes the open issues or pull requests that a command of kind takes.
 func (a *application) completeDiscussions(kind refKind) cobra.CompletionFunc {
 	if kind == pullRef {
-		complete := a.completePulls("open")
-		return func(cmd *cobra.Command, args []string, toComplete string) ([]cobra.Completion, cobra.ShellCompDirective) {
-			if len(args) > 0 {
-				return nil, cobra.ShellCompDirectiveNoFileComp
-			}
-			return complete(cmd, args, toComplete)
-		}
+		return firstArg(a.completePulls("open"))
 	}
-	return a.completeIssueArgs("open", false)
+	return firstArg(a.completeIssues("open"))
 }