Merge branch 'jh/maint-do-not-track-non-branches'

* jh/maint-do-not-track-non-branches:
  branch/checkout --track: Ensure that upstream branch is indeed a branch

diff --cc branch.c
Simple merge

diff --cc t/t3200-branch.sh
Simple merge

diff --cc t/t6040-tracking-info.sh
Simple merge