Sync with 2.17.6

* maint-2.17:
  Git 2.17.6
  unpack_trees(): start with a fresh lstat cache
  run-command: invalidate lstat cache after a command finished
  checkout: fix bug that makes checkout follow symlinks in leading path

diff --cc cache.h
Simple merge

diff --cc compat/mingw.c
Simple merge

diff --cc git-compat-util.h
Simple merge

diff --cc run-command.c
Simple merge

diff --cc t/t0021-conversion.sh
Simple merge

diff --cc unpack-trees.c
Simple merge