update to libgit2 / pygit2 v0.19.0
BuildSpec: improve environment semantics

The existing behaviour of merging spec environment and os.environ in
teh BuildSpec constructor is dubious.  Modify BuildSpec so that the
env attribute represents precisely what lives in the repository and
the os.environ merge is performed as part of the execute process.
test_build: add missing import
BuildSpec: chase more v0.19.0 API changes

Incomplete tests missed breakage due to API change when reading
BuildSpec from repository.  Add this test case and chase the API
change.