Prepare for release v31 (google#1489)

Implement support for repository visibility parameter (google#1471)

Fixes google#1470.

Prepare for release v30.0.0 (google#1456)

Move the GitHub Teams API (google#1400)

Fixes: google#1387.

Implement file deletion for CreateTree (google#1372)

Bump major version to v29 (take 2) (google#1367)

Fixes google#1365.

Bump major version to v29 (google#1344)

Add start and end column to Annotation (google#1241)

Update git_refs.go (google#1240)

Support go1.13 (google#1279)

Closes google#1273.