Merge branch 'jk/checkout-from-tree' into maint

"git checkout $treeish $path", when $path in the index and the
working tree already matched what is in $treeish at the $path,
still overwrote the $path unnecessarily.

* jk/checkout-from-tree:
  checkout $tree: do not throw away unchanged index entries

diff --cc builtin/checkout.c
Simple merge