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

diff --cc builtin-fsck.c
Simple merge

diff --cc builtin-update-index.c
Simple merge

diff --cc cache.h
Simple merge

diff --cc diff-lib.c
Simple merge

diff --cc refs.c
Simple merge

diff --cc sha1_file.c
Simple merge