Refactor, rename phlsys_git.GitClone -> Repo

'GitClone' never sounded right, it's a repository and 'repo' seems to be
a pretty widely used abbreviation.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

manually inspect diff for correct substitutions
git grep for GitClone for missed substitutions
phlsys_git: refactor, rm unused tmprepo_context

Test Plan:
$ ./precommit.sh
$ git grep tmprepo_context
Refactor, rename abdt_git.GitClone -> Repo

'Clone' never sounded right, it's a repository and 'repo' seems to be
a pretty widely used abbreviation.

Test Plan:
$ ./precommit.sh
$ ./testbed/arcyd/acceptance_tests.sh

manually inspect diff for correct substitutions
git grep for Clone for missed substitutions