Merge branch 'sg/dir-trie-fixes'

Code clean-up and a bugfix in the logic used to tell worktree local
and repository global refs apart.

* sg/dir-trie-fixes:
  path.c: don't call the match function without value in trie_find()
  path.c: clarify two field names in 'struct common_dir'
  path.c: mark 'logs/HEAD' in 'common_list' as file
  path.c: clarify trie_find()'s in-code comment
  Documentation: mention more worktree-specific exceptions

diff --cc Documentation/gitrepository-layout.txt
Simple merge

diff --cc path.c
Simple merge