Merge branch 'jc/maint-read-tree-multi'

* jc/maint-read-tree-multi:
  checkout branch: prime cache-tree fully
  read-tree -m A B: prime cache-tree from the switched-to tree
  Move prime_cache_tree() to cache-tree.c
  read-tree A B: do not corrupt cache-tree

diff --cc builtin-checkout.c
Simple merge

diff --cc builtin-read-tree.c
Simple merge