reset [<commit>] paths...: do not mishandle unmerged paths

Because "diff --cached HEAD" showed an incorrect blob object name on the
LHS of the diff, we ended up updating the index entry with bogus value,
not what we read from the tree.

Noticed by John Nowak.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 18 insertions, 2 deletions

diff --git a/builtin/reset.c b/builtin/reset.c
index 98bca04..777e7c6 100644
--- a/builtin/reset.c
+++ b/builtin/reset.c
@@ -162,7 +162,7 @@ static void update_index_from_diff(struct diff_queue_struct *q,
 
 	for (i = 0; i < q->nr; i++) {
 		struct diff_filespec *one = q->queue[i]->one;
-		if (one->mode) {
+		if (one->mode && !is_null_sha1(one->sha1)) {
 			struct cache_entry *ce;
 			ce = make_cache_entry(one->mode, one->sha1, one->path,
 				0, 0);
diff --git a/diff-lib.c b/diff-lib.c
index 9c29293..fd61acb 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -379,7 +379,8 @@ static void do_oneway_diff(struct unpack_trees_options *o,
 	if (cached && idx && ce_stage(idx)) {
 		struct diff_filepair *pair;
 		pair = diff_unmerge(&revs->diffopt, idx->name);
-		fill_filespec(pair->one, idx->sha1, idx->ce_mode);
+		if (tree)
+			fill_filespec(pair->one, tree->sha1, tree->ce_mode);
 		return;
 	}
 
diff --git a/t/t7102-reset.sh b/t/t7102-reset.sh
index f1cfc9a..b096dc8 100755
--- a/t/t7102-reset.sh
+++ b/t/t7102-reset.sh
@@ -429,6 +429,21 @@ test_expect_success '--mixed refreshes the index' '
 	test_i18ncmp expect output
 '
 
+test_expect_success 'resetting specific path that is unmerged' '
+	git rm --cached file2 &&
+	F1=$(git rev-parse HEAD:file1) &&
+	F2=$(git rev-parse HEAD:file2) &&
+	F3=$(git rev-parse HEAD:secondfile) &&
+	{
+		echo "100644 $F1 1	file2" &&
+		echo "100644 $F2 2	file2" &&
+		echo "100644 $F3 3	file2"
+	} | git update-index --index-info &&
+	git ls-files -u &&
+	test_must_fail git reset HEAD file2 &&
+	git diff-index --exit-code --cached HEAD
+'
+
 test_expect_success 'disambiguation (1)' '
 
 	git reset --hard &&