filter: internal git_buf filter handling function

Introduce `git_filter_list__convert_buf` which behaves like the old
implementation of `git_filter_list__apply_data`, where it might move the
input data buffer over into the output data buffer space for efficiency.

This new implementation will do so in a more predictible way, always
freeing the given input buffer (either moving it to the output buffer or
filtering it into the output buffer first).

Convert internal users to it.

src/checkout.c

@@ -2121,7 +2121,7 @@ static int checkout_write_merge(
 		if ((error = git_filter_list__load_ext(
 				&fl, data->repo, NULL, git_buf_cstr(&path_workdir),
 				GIT_FILTER_TO_WORKTREE, &filter_opts)) < 0 ||
-			(error = git_filter_list_apply_to_data(&out_data, fl, &in_data)) < 0)
+			(error = git_filter_list__convert_buf(&out_data, fl, &in_data)) < 0)
 			goto done;
 	} else {
 		out_data.ptr = (char *)result.ptr;

src/diff_file.c

@@ -362,10 +362,7 @@ static int diff_file_content_load_workdir_file(
 	if (!(error = git_futils_readbuffer_fd(&raw, fd, (size_t)fc->file->size))) {
 		git_buf out = GIT_BUF_INIT;
 
-		error = git_filter_list_apply_to_data(&out, fl, &raw);
-
-		if (out.ptr != raw.ptr)
-			git_buf_dispose(&raw);
+		error = git_filter_list__convert_buf(&out, fl, &raw);
 
 		if (!error) {
 			fc->map.len  = out.size;

src/filter.c

@@ -742,6 +742,28 @@ int git_filter_list_apply_to_buffer(
 	return error;
 }
 
+int git_filter_list__convert_buf(
+	git_buf *out,
+	git_filter_list *filters,
+	git_buf *in)
+{
+	int error;
+
+	if (!filters || git_filter_list_length(filters) == 0) {
+		git_buf_swap(out, in);
+		git_buf_dispose(in);
+		return 0;
+	}
+
+	error = git_filter_list_apply_to_buffer(out, filters,
+		in->ptr, in->size);
+
+	if (!error)
+		git_buf_dispose(in);
+
+	return error;
+}
+
 int git_filter_list_apply_to_file(
 	git_buf *out,
 	git_filter_list *filters,

src/filter.h

@@ -35,6 +35,15 @@ extern int git_filter_list__load_ext(
 	git_filter_mode_t mode,
 	git_filter_options *filter_opts);
 
+/*
+ * The given input buffer will be converted to the given output buffer.
+ * The input buffer will be freed (_if_ it was allocated).
+ */
+extern int git_filter_list__convert_buf(
+	git_buf *out,
+	git_filter_list *filters,
+	git_buf *in);
+
 /*
  * Available filters
  */

src/odb.c

@@ -260,9 +260,7 @@ int git_odb__hashfd_filtered(
 	if (!(error = git_futils_readbuffer_fd(&raw, fd, size))) {
 		git_buf post = GIT_BUF_INIT;
 
-		error = git_filter_list_apply_to_data(&post, fl, &raw);
-
-		git_buf_dispose(&raw);
+		error = git_filter_list__convert_buf(&post, fl, &raw);
 
 		if (!error)
 			error = git_odb_hash(out, post.ptr, post.size, type);