Merge branch 'lt/gitlink'

Merge branch 'lt/gitlink'

* lt/gitlink:
  Tests for core subproject support
  Expose subprojects as special files to "git diff" machinery
  Fix some "git ls-files -o" fallout from gitlinks
  Teach "git-read-tree -u" to check out submodules as a directory
  Teach git list-objects logic to not follow gitlinks
  Fix gitlink index entry filesystem matching
  Teach "git-read-tree -u" to check out submodules as a directory
  Teach git list-objects logic not to follow gitlinks
  Don't show gitlink directories when we want "other" files
  Teach git-update-index about gitlinks
  Teach directory traversal about subprojects
  Fix thinko in subproject entry sorting
  Teach core object handling functions about gitlinks
  Teach "fsck" not to follow subproject links
  Add "S_IFDIRLNK" file mode infrastructure for git links
  Add 'resolve_gitlink_ref()' helper function
  Avoid overflowing name buffer in deep directory structures
  diff-lib: use ce_mode_from_stat() rather than messing with modes manually