[PATCH 2/9] Introduce CE_NO_CHECKOUT bit

@@ -171,6 +171,39 @@ the reflog for HEAD where you were, e.g.
 $ git log -g -2 HEAD
 ------------
 
+Sparse checkout
+---------------
+
+Normally when you checkout a branch, your working directory
+will be fully populated. In some situations, you just need to
+work on certain files, no full checkout is needed. Sparse
+checkout is a mode that limits the checkout area according to your
+needs. With sparse checkout, you can work on a single file, a
+collection of files, a subdirectory or a collection of separated
+subdirectories.
+
+Because sparse checkout uses a new index format, it will be
+incompatible with git prior to 1.6.0 regarding worktree operations.
+Operations that only need access to the repository itself, such as
+clone, push, or pull/fetch from another (normal) repository... should
+not be affected by sparse checkout.
+
+In sparse checkout mode, checkout status of every files in your
+working directory will be recorded in index. If a file is marked
+"no-checkout", it means that file is not needed to be present in
+working directory by user or any git command. When a new file is added
+to index, it will be marked "checkout" unless sparse patterns are
+applied.  Unmerged files are always "checkout". When you checkout new
+files using "git checkout <file>" they will be automatically marked
+"checkout". Other commands such as "git apply" can also checkout new
+files if they are needed.
+
+"No-checkout" status is very similar to "assume-unchanged bit"
+(see linkgit:git-update-index[1]). The main difference between them
+is "assume unchanged" bit just ignores corresponding files in working
+directory while sparse checkout goes a bit farther, remove those files
+when it is safe to do so.
+
 
 EXAMPLES
 --------

@@ -170,10 +170,11 @@ struct cache_entry {
 /*
  * Extended on-disk flags
  */
+#define CE_NO_CHECKOUT 0x40000000
 /* CE_EXTENDED2 is for future extension */
 #define CE_EXTENDED2 0x80000000
 
-#define CE_EXTENDED_FLAGS (0)
+#define CE_EXTENDED_FLAGS (CE_NO_CHECKOUT)
 
 /*
  * Safeguard to avoid saving wrong flags:

@@ -185,6 +186,9 @@ struct cache_entry {
 #error "CE_EXTENDED_FLAGS out of range"
 #endif
 
+/* "Assume unchanged" mask */
+#define CE_VALID_MASK (CE_VALID | CE_NO_CHECKOUT)
+
 /*
  * Copy the sha1 and stat state of a cache entry from one to
  * another. But we never change the name, or the hash state!

@@ -222,6 +226,10 @@ static inline size_t ce_namelen(const struct cache_entry *ce)
 			    ondisk_cache_entry_size(ce_namelen(ce)))
 #define ce_stage(ce) ((CE_STAGEMASK & (ce)->ce_flags) >> CE_STAGESHIFT)
 #define ce_uptodate(ce) ((ce)->ce_flags & CE_UPTODATE)
+#define ce_no_checkout(ce) ((ce)->ce_flags & CE_NO_CHECKOUT)
+#define ce_checkout(ce) (!ce_no_checkout(ce))
+#define ce_mark_no_checkout(ce) ((ce)->ce_flags |= CE_NO_CHECKOUT)
+#define ce_mark_checkout(ce) ((ce)->ce_flags &= ~CE_NO_CHECKOUT)
 #define ce_mark_uptodate(ce) ((ce)->ce_flags |= CE_UPTODATE)
 
 #define ce_permissions(mode) (((mode) & 0100) ? 0755 : 0644)

@@ -254,7 +254,7 @@ int ie_match_stat(const struct index_state *istate,
 	 * If it's marked as always valid in the index, it's
 	 * valid whatever the checked-out copy says.
 	 */
-	if (!ignore_valid && (ce->ce_flags & CE_VALID))
+	if (!ignore_valid && (ce->ce_flags & CE_VALID_MASK))
 		return 0;
 
 	changed = ce_match_stat_basic(ce, st);

@@ -962,10 +962,10 @@ static struct cache_entry *refresh_cache_ent(struct index_state *istate,
 		return ce;
 
 	/*
-	 * CE_VALID means the user promised us that the change to
+	 * CE_VALID_MASK means the user promised us that the change to
 	 * the work tree does not matter and told us not to worry.
 	 */
-	if (!ignore_valid && (ce->ce_flags & CE_VALID)) {
+	if (!ignore_valid && (ce->ce_flags & CE_VALID_MASK)) {
 		ce_mark_uptodate(ce);
 		return ce;
 	}