Do not return all files when checking changed files from last commit

[rafeca]

Summary

#6732 improved the commands used to check the files that have checked for both Mercurial and Git repositories, but caused a bug on Mercurial repositories.

Turns out that executing hg status -A returns every single file on the repository, which is wrong (and extreeeemely slow for huge repos like FB's one 😅).

This PR changes the arguments passed to the hg command when lastCommit is true to fix the problem.

Test plan

jest-changed-files does not have any tests (which is understandable since testing its functionality is not straightforward), so I've done some manual testing:

1. Execute manually hg status -amnu --change . <some_folders> on a Mercurial repository and check that it always returns the changed files from the current commit.
2. Patch this PR into our internal codebase with a WIP version of Jest v24.0.0-alpha.0 and check that our continuous integration tests pass correctly.

[cpojer]

Does this also print added files?

[rafeca]

Yes, this should print added files (on the last commit)

[github-actions]

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