Originally, moving a <source> directory which is not on-disk due
to its existence outside of sparse-checkout cone, "giv mv" command
errors out with "bad source".

Add a helper check_dir_in_index() function to see if a directory
name exists in the index. Also add a SKIP_WORKTREE_DIR bit to mark
such directories.

Change the checking logic, so that such <source> directory makes
"giv mv" command warns with "advise_on_updating_sparse_paths()"
instead of "bad source"; also user now can supply a "--sparse" flag so
this operation can be carried out successfully.

Helped-by: Victoria Dye [off-list ref]
Helped-by: Derrick Stolee [off-list ref]
Signed-off-by: Shaoxuan Yuan <redacted>
---
 builtin/mv.c                  | 50 ++++++++++++++++++++++++++++++-----
 t/t7002-mv-sparse-checkout.sh |  4 +--
 2 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/builtin/mv.c b/builtin/mv.c
index aa29da4337..b5d0d8ef4f 100644
--- a/builtin/mv.c
+++ b/builtin/mv.c

@@ -25,6 +25,7 @@ enum update_mode {
 	WORKING_DIRECTORY = (1 << 1),
 	INDEX = (1 << 2),
 	SPARSE = (1 << 3),
+	SKIP_WORKTREE_DIR = (1 << 4),
 };
 
 #define DUP_BASENAME 1

@@ -123,6 +124,36 @@ static int index_range_of_same_dir(const char *src, int length,
 	return last - first;
 }
 
+/*
+ * Check if an out-of-cone directory should be in the index. Imagine this case
+ * that all the files under a directory are marked with 'CE_SKIP_WORKTREE' bit
+ * and thus the directory is sparsified.
+ *
+ * Return 0 if such directory exist (i.e. with any of its contained files not
+ * marked with CE_SKIP_WORKTREE, the directory would be present in working tree).
+ * Return 1 otherwise.
+ */