[PATCH 2/3] diff-lib.c: enable --shift-ita in index_differs_from()

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

This function is basically "git diff --cached HEAD", It has three
callers:

 - One in builtin/commit.c, which uses it to determine if the index is
   different from HEAD and go ahead making a new commit.

 - Two in sequencer.c, which use it to see if the index is dirty.

In the first case, if ita entries are present, index_differs_from() may
report "dirty". However at tree creation phase, ita entries are dropped
and the result tree may look exactly the same as HEAD (assuming that
nothing else is changed in index). This is what we need index_differs_from()
for, to catch new empty commits. Enabling shift_ita in index_differs_from()
fixes this.

In the second case, the presence of ita entries are enough to say the
index is dirty and not continue on. Make an explicit check for that
before comparing index against HEAD (whether --shift-ita is present is
irrelevant)

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 cache.h               |  1 +
 diff-lib.c            |  1 +
 read-cache.c          | 10 ++++++++++
 sequencer.c           |  5 +++--
 t/t2203-add-intent.sh | 11 +++++++++++
 5 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/cache.h b/cache.h
index d0494c8..1ddd515 100644
--- a/cache.h
+++ b/cache.h
@@ -561,6 +561,7 @@ extern int do_read_index(struct index_state *istate, const char *path,
 extern int read_index_from(struct index_state *, const char *path);
 extern int is_index_unborn(struct index_state *);
 extern int read_index_unmerged(struct index_state *);
+extern int has_ita_entries(struct index_state *);
 #define COMMIT_LOCK		(1 << 0)
 #define CLOSE_LOCK		(1 << 1)
 extern int write_locked_index(struct index_state *, struct lock_file *lock, unsigned flags);
diff --git a/diff-lib.c b/diff-lib.c
index 62d67c8..ea55ee2 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -545,6 +545,7 @@ int index_differs_from(const char *def, int diff_flags)
 	DIFF_OPT_SET(&rev.diffopt, QUICK);
 	DIFF_OPT_SET(&rev.diffopt, EXIT_WITH_STATUS);
 	rev.diffopt.flags |= diff_flags;
+	rev.diffopt.shift_ita = 1;
 	run_diff_index(&rev, 1);
 	if (rev.pending.alloc)
 		free(rev.pending.objects);
diff --git a/read-cache.c b/read-cache.c
index 31eddec..f6a5f61 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1674,6 +1674,16 @@ int is_index_unborn(struct index_state *istate)
 	return (!istate->cache_nr && !istate->timestamp.sec);
 }
 
+int has_ita_entries(struct index_state *istate)
+{
+	int i;
+
+	for (i = 0; i < istate->cache_nr; i++)
+		if (ce_intent_to_add(istate->cache[i]))
+			return 1;
+	return 0;
+}
+
 int discard_index(struct index_state *istate)
 {
 	int i;
diff --git a/sequencer.c b/sequencer.c
index eec8a60..10cded0 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -469,7 +469,8 @@ static int do_pick_commit(struct commit *commit, struct replay_opts *opts)
 		unborn = get_sha1("HEAD", head);
 		if (unborn)
 			hashcpy(head, EMPTY_TREE_SHA1_BIN);
-		if (index_differs_from(unborn ? EMPTY_TREE_SHA1_HEX : "HEAD", 0))
+		if (has_ita_entries(&the_index) ||
+		    index_differs_from(unborn ? EMPTY_TREE_SHA1_HEX : "HEAD", 0))
 			return error_dirty_index(opts);
 	}
 	discard_cache();
@@ -1064,7 +1065,7 @@ static int sequencer_continue(struct replay_opts *opts)
 		if (ret)
 			return ret;
 	}
-	if (index_differs_from("HEAD", 0))
+	if (has_ita_entries(&the_index) || index_differs_from("HEAD", 0))
 		return error_dirty_index(opts);
 	todo_list = todo_list->next;
 	return pick_commits(todo_list, opts);
diff --git a/t/t2203-add-intent.sh b/t/t2203-add-intent.sh
index c6a4648..aa06415 100755
--- a/t/t2203-add-intent.sh
+++ b/t/t2203-add-intent.sh
@@ -129,5 +129,16 @@ test_expect_success 'cache-tree does skip dir that becomes empty' '
 	)
 '
 
+test_expect_success 'commit: ita entries ignored in empty commit check' '
+	git init empty-subsequent-commit &&
+	(
+		cd empty-subsequent-commit &&
+		test_commit one &&
+		: >two &&
+		git add -N two &&
+		test_must_fail git commit -m nothing-new-here
+	)
+'
+
 test_done
 
-- 
2.8.2.524.g6ff3d78