Introduce "sparse checkout"

With skip-worktree bit, you can manually set it to unwanted files,
then remove them: you would have the so-called sparse checkout. The
disadvantages are:

 - Porcelain tools are not aware of this. Everytime you do an
   operation that may update working directory, skip-worktree may be
   cleared out. You have to set them again.

 - You still have to remove skip-worktree'd files manually, which is
   boring and ineffective.

These will be addressed in the following patches. This patch gives an
idea what is "sparse checkout" in Documentation/git-read-tree.txt.
This file is chosen instead of git-checkout.txt because it is quite
technical and user-unfriendly. I'd expect git-checkout.txt to have
something when Porcelain support is done.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-read-tree.txt

@@ -360,6 +360,50 @@ middle of doing, and when your working tree is ready (i.e. you
 have finished your work-in-progress), attempt the merge again.
 
 
+Sparse checkout
+---------------
+
+"Sparse checkout" allows to sparsely populate working directory.
+It uses skip-worktree bit (see linkgit:git-update-index[1]) to tell
+Git whether a file on working directory is worth looking at.
+
+"git read-tree" and other merge-based commands ("git merge", "git
+checkout"...) can help maintaining skip-worktree bitmap and working
+directory update. `$GIT_DIR/info/sparse-checkout` is used to
+define the skip-worktree reference bitmap. When "git read-tree" needs
+to update working directory, it will reset skip-worktree bit in index
+based on this file, which uses the same syntax as .gitignore files.
+If an entry matches a pattern in this file, skip-worktree will be
+set on that entry. Otherwise, skip-worktree will be unset.
+
+Then it compares the new skip-worktree value with the previous one. If
+skip-worktree turns from unset to set, it will add the corresponding
+file back. If it turns from set to unset, that file will be removed.
+
+While `$GIT_DIR/info/sparse-checkout` is usually used to specify what
+files are in. You can also specify what files are _not_ in, using
+negate patterns. For example, to remove file "unwanted":
+
+----------------
+*
+!unwanted
+----------------
+
+Another tricky thing is fully repopulating working directory when you
+no longer want sparse checkout. You cannot just disable "sparse
+checkout" because skip-worktree are still in the index and you working
+directory is still sparsely populated. You should re-populate working
+directory with the `$GIT_DIR/info/sparse-checkout` file content as
+follows:
+
+----------------
+*
+----------------
+
+Then you can disable sparse checkout. Sparse checkout support in "git
+read-tree" and similar commands is disabled by default.
+
+
 SEE ALSO
 --------
 linkgit:git-write-tree[1]; linkgit:git-ls-files[1];