Update "git checkout foo" that DWIMs the intended "upstream" and
turns it into "git checkout -t -b foo remotes/origin/foo" to
correctly take existing remote definitions into account.

The remote "origin" may be what uniquely map its own branch to
remotes/some/where/foo but that some/where may not be "origin".

* jh/checkout-auto-tracking:
  glossary: Update and rephrase the definition of a remote-tracking branch
  branch.c: Validate tracking branches with refspecs instead of refs/remotes/*
  t9114.2: Don't use --track option against "svn-remote"-tracking branches
  t7201.24: Add refspec to keep --track working
  t3200.39: tracking setup should fail if there is no matching refspec.
  checkout: Use remote refspecs when DWIMming tracking branches
  t2024: Show failure to use refspec when DWIMming remote branch names
  t2024: Add tests verifying current DWIM behavior of 'git checkout <branch>'