"git checkout" and "git worktree add" learned to honor
checkout.defaultRemote when auto-vivifying a local branch out of a
remote tracking branch in a repository with multiple remotes that
have tracking branches that share the same names.

* ab/checkout-default-remote:
  checkout & worktree: introduce checkout.defaultRemote
  checkout: add advice for ambiguous "checkout <branch>"
  builtin/checkout.c: use "ret" variable for return
  checkout: pass the "num_matches" up to callers
  checkout.c: change "unique" member to "num_matches"
  checkout.c: introduce an *_INIT macro
  checkout.h: wrap the arguments to unique_tracking_name()
  checkout tests: index should be clean after dwim checkout