Use get-branch instead of list-branches (with a limit:100)

[jehiah]

When folks don't strictly manage their outstanding branches and pass the 100 limit, git-open-pull can't always find the branch on github to open-pull against (depending on sorting).

Github now has a get-branch endpoint that should be used instead of list-branches

https://developer.github.com/v3/repos/branches/#get-branch

git-open-pull/src/cmd/git-open-pull/main.go

Line 138 in 5ca44a6

branches, _, err := client.Repositories.ListBranches(ctx, settings.User, settings.BaseRepo, &github.ListOptions{PerPage: 100})

cc: @kpurdon