amend: add tests for amending only some files from commit to be amended We do not have robust enough tests for scenarios where only some files in a changeset are amended. This presents an interesting scenario because the working copy could have modified versions of the remaining files in the pre-amend changeset. Therefore, I have added some tests to ensure that amend behaves as expected in these scenarios. Test Plan: Ensured that the test "test-commit-amend.t" passes. Differential Revision: https://phab.mercurial-scm.org/D596

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