Run git_add when extracting localizations

I noticed this edge case in the 21.1 release cycle, where we sent a new
file, the `InfoPlist.strings` for the Jetpack save to draft extension,
to GlotPress for translation and the automation failed because it
downloaded the new localizations and could not commit them.

Author: mokagio

fastlane/lanes/localization.rb

@@ -247,6 +247,9 @@
       source_parent_dir: parent_dir_for_lprojs,
       target_original_files: MANUALLY_MAINTAINED_STRINGS_FILES
     )
+    # Manually add files in case there are entirely new localization files.
+    # Fastlane's `git_commit` can only commit changes to existing files.
+    git_add(path: modified_files, shell_escape: false)
     git_commit(
       path: modified_files,
       message: 'Update app translations – Other `.strings`',