Merge branch 'jh/filter-empty-contents' into maint

The clean/smudge interface did not work well when filtering an
empty contents (failed and then passed the empty input through).
It can be argued that a filter that produces anything but empty for
an empty input is nonsense, but if the user wants to do strange
things, then why not?

* jh/filter-empty-contents:
  sha1_file: pass empty buffer to index empty file

sha1_file.c

@@ -3202,7 +3202,7 @@ static int index_core(unsigned char *sha1, int fd, size_t size,
 	int ret;
 
 	if (!size) {
-		ret = index_mem(sha1, NULL, size, type, path, flags);
+		ret = index_mem(sha1, "", size, type, path, flags);
 	} else if (size <= SMALL_FILE_SIZE) {
 		char *buf = xmalloc(size);
 		if (size == read_in_full(fd, buf, size))

t/t0021-conversion.sh

@@ -216,4 +216,30 @@ test_expect_success EXPENSIVE 'filter large file' '
 	! test -s err
 '
 
+test_expect_success "filter: clean empty file" '
+	git config filter.in-repo-header.clean  "echo cleaned && cat" &&
+	git config filter.in-repo-header.smudge "sed 1d" &&
+
+	echo "empty-in-worktree    filter=in-repo-header" >>.gitattributes &&
+	>empty-in-worktree &&
+
+	echo cleaned >expected &&
+	git add empty-in-worktree &&
+	git show :empty-in-worktree >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success "filter: smudge empty file" '
+	git config filter.empty-in-repo.clean "cat >/dev/null" &&
+	git config filter.empty-in-repo.smudge "echo smudged && cat" &&
+
+	echo "empty-in-repo filter=empty-in-repo" >>.gitattributes &&
+	echo dead data walking >empty-in-repo &&
+	git add empty-in-repo &&
+
+	echo smudged >expected &&
+	git checkout-index --prefix=filtered- empty-in-repo &&
+	test_cmp expected filtered-empty-in-repo
+'
+
 test_done