Merge branch 'maint'

* maint:
  unpack_trees(): protect the handcrafted in-core index from read_cache()
  git-p4: Fix one-liner in p4_write_pipe function.
  Completion: add missing '=' for 'diff --diff-filter'
  Fix 'git help help'

diff --cc cache.h
Simple merge

diff --cc read-cache.c
Simple merge