phlgitu_fixture: workers fetch, checkout at init

Simplify test code by moving a common initialisation step inside the
fixture itself.

Test Plan:
$ ./precommit.sh
Add phlgit_fetch__t: rudimentary tests

Exercise the 'branch bomb' avoidance of 'prune_safe'.

Test Plan:
$ ./precommit.sh
WIP: phlgit_fetch: add support for fetching with a fetchspec