unpack-trees(): ignore worktree check outside checkout area

verify_absent() and verify_uptodate() are used to ensure worktree
is safe to be updated, then CE_REMOVE or CE_UPDATE will be set.
Finally check_updates() bases on CE_REMOVE, CE_UPDATE and the
recently added CE_WT_REMOVE to update working directory accordingly.

The entries that are checked may eventually be left out of checkout
area (done later in apply_sparse_checkout()). We don't want to update
outside checkout area. This patch teaches Git to assume "good",
skip these checks when it's sure those entries will be outside checkout
area, and clear CE_REMOVE|CE_UPDATE that could be set due to this
assumption.

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

unpack-trees.c

@@ -505,6 +505,14 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
 				ret = -1;
 				goto done;
 			}
+			/*
+			 * Merge strategies may set CE_UPDATE|CE_REMOVE outside checkout
+			 * area as a result of ce_skip_worktree() shortcuts in
+			 * verify_absent() and verify_uptodate(). Clear them.
+			 */
+			if (ce_skip_worktree(ce))
+				ce->ce_flags &= ~(CE_UPDATE | CE_REMOVE);
+
 		}
 	}
 
@@ -577,6 +585,8 @@ static int verify_uptodate_1(struct cache_entry *ce,
 static int verify_uptodate(struct cache_entry *ce,
 			   struct unpack_trees_options *o)
 {
+	if (!o->skip_sparse_checkout && will_have_skip_worktree(ce, o))
+		return 0;
 	return verify_uptodate_1(ce, o, ERRORMSG(o, not_uptodate_file));
 }
 
@@ -776,6 +786,8 @@ static int verify_absent_1(struct cache_entry *ce, const char *action,
 static int verify_absent(struct cache_entry *ce, const char *action,
 			 struct unpack_trees_options *o)
 {
+	if (!o->skip_sparse_checkout && will_have_skip_worktree(ce, o))
+		return 0;
 	return verify_absent_1(ce, action, o, ERRORMSG(o, would_lose_untracked));
 }