Build looks for a "master" branch, but "main" is GitHub's new default

[antoineco]

During a build, I witnessed the following error:

{
  "level":"error",
  "ts":1604588951.297186,
  "caller":"git/git.go:54",
  "msg":"Error running git [fetch --recurse-submodules=yes --depth=1 origin --update-head-ok --force master]: exit status 128
         fatal: couldn't find remote ref master",
  "stacktrace":"github.com/tektoncd/pipeline/pkg/git.run
        github.com/tektoncd/pipeline/pkg/git/git.go:54
github.com/tektoncd/pipeline/pkg/git.Fetch
        github.com/tektoncd/pipeline/pkg/git/git.go:145
main.main
        github.com/tektoncd/pipeline/cmd/git-init/main.go:52
runtime.main
        runtime/proc.go:203"
}

 {
  "level":"fatal",
  "ts":1604588951.2973213,
  "caller":"git-init/main.go:53",
  "msg":"Error fetching git repository: failed to fetch [master]: exit status 128","stacktrace":"main.main
        github.com/tektoncd/pipeline/cmd/git-init/main.go:53
runtime.main
        runtime/proc.go:203"
}