`gh stack link` does not respect the default branch of a repository

[tylerjohnst]

When linking an existing stack with gh link 1 2 3 and --base is not given, it defaults to main not the default repository branch. As a result, the first PR of the stack is retargeted to main and currently there is no easy way to change the base of the stack.

[skarim]

link currently has a hardcoded default to main:

gh-stack/cmd/link.go

Line 80 in 38aba1b

cmd.Flags().StringVar(&opts.base, "base", "main", "Base branch for the bottom of the stack")

but it should follow the same dynamic base branch resolution that is used in init:

gh-stack/cmd/init.go

Line 75 in 38aba1b

trunk, err = git.DefaultBranch()