Minor refactor of checkout
Don't import log
Use explicit binary dir for hub
Bump dependencies.

Updated go-sawyer, go-octokit and pflag
Make sure charset is there when validating HTTP_ACCEPT
The head of the repo in a pull request can be nil
Fix merge.feature as well
Merge pull request #534 from github/checkout_compat

gh: Fix `merge.feature` and `checkout.feature`