Refs T13665
In the update to D21716 the str_starts_with function was added which is only available in PHP 8. This changes it to use strncmp() which is compatible back to PHP 4.
Additionally fixed running into an error when trying to run arc amend on a commit which already matches the content known in Phabricator. Mercurial throws an error when running amend without file changes and using the exact same commit message it already has.