Merge branch 'jc/checkout-no-op-switch-errors'

"git checkout --ours" (no other arguments) complained that the
option is incompatible with branch switching, which is technically
correct, but found confusing by some users.  It now says that the
user needs to give pathspec to specify what paths to checkout.

* jc/checkout-no-op-switch-errors:
  checkout: special case error messages during noop switching

diff --cc builtin/checkout.c
Simple merge

diff --cc t/t7201-co.sh
Simple merge