[WIP v1 3/4] mv: add advise_to_reapply hint for moving file into cone

[Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>]

Originally, the SKIP_WORKTREE bit is not removed when moving an out-of-cone
file into sparse cone, thus the moved file does not show up in the working tree.
Hint the user to use "git sparse-checkout reapply" to reapply the sparsity rules
to the working tree, by which the SKIP_WORKTREE bit is removed.

Signed-off-by: Shaoxuan Yuan <shaoxuan.yuan02@gmail.com>
---
I offered this solution becasue I'm not sure how to turn a cache_entry's 
ce_flags back to a non-sparse state. I tried directly set it to 0 like this:

	ce->ce_flags = 0;

But the behavior after this seems undefined. The file still won't show up
in the working tree.

And I found that "git sparse-checkout reapply" seems to be a nice fit for the
job. But I guess if there is a way (there must be but I don't know) to do it
direcly in the code, that could also be nice.

 builtin/mv.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/builtin/mv.c b/builtin/mv.c
index 9da9205e01..5f511fb8da 100644
--- a/builtin/mv.c
+++ b/builtin/mv.c
@@ -138,6 +138,7 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 {
 	int i, flags, gitmodules_modified = 0;
 	int verbose = 0, show_only = 0, force = 0, ignore_errors = 0, ignore_sparse = 0;
+	int advise_to_reapply = 0;
 	struct option builtin_mv_options[] = {
 		OPT__VERBOSE(&verbose, N_("be verbose")),
 		OPT__DRY_RUN(&show_only, N_("dry run")),
@@ -383,6 +384,11 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 		pos = cache_name_pos(src, strlen(src));
 		assert(pos >= 0);
 		rename_cache_entry_at(pos, dst);
+		if (!advise_to_reapply &&
+			!path_in_sparse_checkout(src, &the_index) &&
+			path_in_sparse_checkout(dst, &the_index)) {
+				advise_to_reapply = 1;
+			}
 	}
 
 	if (gitmodules_modified)
@@ -392,6 +398,9 @@ int cmd_mv(int argc, const char **argv, const char *prefix)
 			       COMMIT_LOCK | SKIP_IF_UNCHANGED))
 		die(_("Unable to write new index file"));
 
+	if (advise_to_reapply)
+		printf(_("Please use \"git sparse-checkout reapply\" to reapply the sparsity.\n"));
+
 	string_list_clear(&src_for_dst, 0);
 	UNLEAK(source);
 	UNLEAK(dest_path);
-- 
2.35.1