Merge branch 'js/realpath-pathdup-fix'

Git v2.12 was shipped with an embarrassing breakage where various
operations that verify paths given from the user stopped dying when
seeing an issue, and instead later triggering segfault.

* js/realpath-pathdup-fix:
  real_pathdup(): fix callsites that wanted it to die on error
  t1501: demonstrate NULL pointer access with invalid GIT_WORK_TREE

diff --cc cache.h
Simple merge

diff --cc setup.c
Simple merge

diff --cc worktree.c
Simple merge