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/repo_mutations.go

@@ -284,7 +284,7 @@ $ tea repo clone alice/project -- --branch develop --recurse-submodules`,
 	command.Flags().IntVarP(&opts.depth, "depth", "d", 0, "Number of commits to fetch (0: full history)")
 	command.Flags().StringVarP(&opts.upstream, "upstream-remote-name", "u", "upstream", "Remote name for the parent of a fork")
 	command.Flags().BoolVar(&noUpstream, "no-upstream", false, "Do not add a remote for the parent of a fork")
-	command.ValidArgsFunction = a.completeRepos()
+	command.ValidArgsFunction = pathArgs(1, false, completeDirectories, a.completeRepos())
 	return command
 }