Merge branch 'bw/attr'

Merge branch 'bw/attr'

The gitattributes machinery is being taught to work better in a
multi-threaded environment.

* bw/attr: (27 commits)
  attr: reformat git_attr_set_direction() function
  attr: push the bare repo check into read_attr()
  attr: store attribute stack in attr_check structure
  attr: tighten const correctness with git_attr and match_attr
  attr: remove maybe-real, maybe-macro from git_attr
  attr: eliminate global check_all_attr array
  attr: use hashmap for attribute dictionary
  attr: change validity check for attribute names to use positive logic
  attr: pass struct attr_check to collect_some_attrs
  attr: retire git_check_attrs() API
  attr: convert git_check_attrs() callers to use the new API
  attr: convert git_all_attrs() to use "struct attr_check"
  attr: (re)introduce git_check_attr() and struct attr_check
  attr: rename function and struct related to checking attributes
  attr.c: outline the future plans by heavily commenting
  Documentation: fix a typo
  attr.c: add push_stack() helper
  attr: support quoting pathname patterns in C style
  attr.c: plug small leak in parse_attr_line()
  attr.c: tighten constness around "git_attr" structure
  ...