Merge branch 'jx/l10n-workflow-change'

A workflow change for translators are being proposed.

* jx/l10n-workflow-change:
  l10n: Document the new l10n workflow
  Makefile: add "po-init" rule to initialize po/XX.po
  Makefile: add "po-update" rule to update po/XX.po
  po/git.pot: don't check in result of "make pot"
  po/git.pot: this is now a generated file
  Makefile: remove duplicate and unwanted files in FOUND_SOURCE_FILES
  i18n CI: stop allowing non-ASCII source messages in po/git.pot
  Makefile: have "make pot" not "reset --hard"
  Makefile: generate "po/git.pot" from stable LOCALIZED_C
  Makefile: sort source files before feeding to xgettext

diff --cc Makefile
Simple merge

diff --cc builtin/submodule--helper.c
Simple merge