Parse commit from packed refs if not available in refs dir.

[chasdevs]

In circle-ci I was getting failures due to their git clone process not providing a reference in .git/refs/heads/master. If that is not available, get the commit hash from .git/packed-refs.

[coveralls]

Coverage increased (+0.5%) to 95.77% when pulling 06f646f on cdeveas:parse-commit-from-packed-refs into e476964 on nickmerwin:master.

[coveralls]

Coverage increased (+0.5%) to 95.77% when pulling c84d09e on cdeveas:parse-commit-from-packed-refs into e476964 on nickmerwin:master.

[coveralls]

Coverage increased (+0.5%) to 95.77% when pulling c84d09e on cdeveas:parse-commit-from-packed-refs into e476964 on nickmerwin:master.

[coveralls]

Coverage increased (+0.5%) to 95.77% when pulling c84d09e on cdeveas:parse-commit-from-packed-refs into e476964 on nickmerwin:master.

[coveralls]

Coverage increased (+0.5%) to 95.77% when pulling 091dae2 on cdeveas:parse-commit-from-packed-refs into e476964 on nickmerwin:master.

[nickmerwin]

This looks great, thanks @cdeveas!