Merge branch 'rs/fsck-duplicate-names-in-trees'

The check in "git fsck" to ensure that the tree objects are sorted
still had corner cases it missed unsorted entries.

* rs/fsck-duplicate-names-in-trees:
  fsck: detect more in-tree d/f conflicts
  t1450: demonstrate undetected in-tree d/f conflict
  t1450: increase test coverage of in-tree d/f detection
  fsck: fix a typo in a comment

diff --cc fsck.c
Simple merge

diff --cc t/t1450-fsck.sh
Simple merge