gettext:regenerate task fails when a locale file has been removed

Summary

When a locale file is deleted, the git ignored files (gitlab.edit.po and gitlab.po.timestamp) remain on local dev environments and the gettext:regenerate task will fail to reset the .po files. This means there are a lot of changes to files that don't need to be committed. This is because the git checkout command fails at the end of the rake task: https://gitlab.com/gitlab-org/gitlab/blob/421ac63d/lib/tasks/gettext.rake#L41-41

Steps to reproduce

1. Create a new locale/<language-code>/gitlab.po file
2. run rails gettext:regenerate
3. Delete the file created in the first step
4. run rails gettext:regenerate

What is the current bug behavior?

All locale files have uncommitted changes

What is the expected correct behavior?

Only the gitlab.pot file has uncommitted changes

Proposed fix

• Only checkout files that actually exist at the end of the rake task. Done in !41473 (merged)
• At the start of the task, force remove locale folders (ie locale/ml_IN) if the folder does not contain a gitlab.po file.

Workaround

Manually delete the entire locale folder that no longer contains a gitlab.po file.

For example:

$ rm -rf locale/ml_IN/