Merge branch 'dt/name-hash-dir-entry-fix' into maint

The name-hash subsystem that is used to cope with case insensitive
filesystems keeps track of directories and their on-filesystem
cases for all the paths in the index by holding a pointer to a
randomly chosen cache entry that is inside the directory (for its
ce->ce_name component).  This pointer was not updated even when the
cache entry was removed from the index, leading to use after free.
This was fixed by recording the path for each directory instead of
borrowing cache entries and restructuring the API somewhat.

* dt/name-hash-dir-entry-fix:
  name-hash: don't reuse cache_entry in dir_entry

diff --cc cache.h
Simple merge

diff --cc dir.c
Simple merge

diff --cc read-cache.c
Simple merge