diff: add a notify callback to `git_diff__from_iterators`

The callback will be called for each file, just before the `git_delta_t` gets inserted into the diff list.

When the callback:
- returns < 0, the diff process will be aborted
- returns > 0, the delta will not be inserted into the diff list, but the diff process continues
- returns 0, the delta is inserted into the diff list, and the diff process continues

1 files changed, 5 insertions, 0 deletions

diff --git a/tests-clar/diff/diff_helpers.h b/tests-clar/diff/diff_helpers.h
index 49c265285..a43847b79 100644
--- a/tests-clar/diff/diff_helpers.h
+++ b/tests-clar/diff/diff_helpers.h
@@ -20,6 +20,11 @@ typedef struct {
 	int line_dels;
 } diff_expects;
 
+typedef struct {
+	const char *path;
+	const char *matched_pathspec;
+} notify_expected;
+
 extern int diff_file_cb(
 	const git_diff_delta *delta,
 	float progress,