Parallelize locale merging in mix gettext.merge (#436)

Benchmark on a 16-locale / 11-domain Phoenix app (160 PO files, ~4,700
msgids, Elixir 1.20.1 / OTP 29, Apple Silicon):

    mix gettext.merge priv/gettext --no-fuzzy

    before: 9.4s wall (55% CPU)
    after:  1.9-2.4s wall (341-381% CPU)

~4.9x speedup with byte-identical output files (clean git status over
committed PO state after both runs). The win scales with locale count.

Author: oliver-kriska

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## Unreleased
+
+  * Parallelize per-locale merging in `mix gettext.merge`.
+
 ## v1.0.2
 
   * Only skip manifest removal on Elixir v1.19.3+

lib/mix/tasks/gettext.merge.ex

@@ -191,9 +191,17 @@ defmodule Mix.Tasks.Gettext.Merge do
   end
 
   defp merge_all_locale_dirs(pot_dir, opts, gettext_config) do
-    for locale <- File.ls!(pot_dir), File.dir?(Path.join(pot_dir, locale)) do
-      merge_dirs(locale_dir(pot_dir, locale), pot_dir, locale, opts, gettext_config)
-    end
+    pot_dir
+    |> File.ls!()
+    |> Enum.filter(&File.dir?(Path.join(pot_dir, &1)))
+    |> Task.async_stream(
+      fn locale ->
+        merge_dirs(locale_dir(pot_dir, locale), pot_dir, locale, opts, gettext_config)
+      end,
+      ordered: false,
+      timeout: :infinity
+    )
+    |> Stream.run()
   end
 
   def locale_dir(pot_dir, locale) do