Add phlgit_fetch__t: rudimentary tests

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

Test Plan:
$ ./precommit.sh
phlgit_fetch: add support for fetching with spec

It can be useful sometimes to customize what gets fetched from a remote,
e.g. when fetching the 'refs/notes' namespace locally.

Supplying a 'refspec' directly to 'git fetch' is one of the ways to
achieve this end.

Add tests to verify that fetching a non-existant namespace is ok.

Test Plan:
$ ./precommit.sh
WIP: Arcyd: simplify fetching to single call
fixup! WIP: Arcyd: simplify fetching to single call
arcyd/acceptance_tests: fix test name