Merge branch 'jt/batch-fetch-blobs-in-diff'

While running "git diff" in a lazy clone, we can upfront know which
missing blobs we will need, instead of waiting for the on-demand
machinery to discover them one by one.  Aim to achieve better
performance by batching the request for these promised blobs.

* jt/batch-fetch-blobs-in-diff:
  diff: batch fetching of missing blobs
  sha1-file: support OBJECT_INFO_FOR_PREFETCH

object-store.h

@@ -280,6 +280,12 @@ struct object_info {
 #define OBJECT_INFO_QUICK 8
 /* Do not check loose object */
 #define OBJECT_INFO_IGNORE_LOOSE 16
+/*
+ * Do not attempt to fetch the object if missing (even if fetch_is_missing is
+ * nonzero). This is meant for bulk prefetching of missing blobs in a partial
+ * clone. Implies OBJECT_INFO_QUICK.
+ */
+#define OBJECT_INFO_FOR_PREFETCH (32 + OBJECT_INFO_QUICK)
 
 int oid_object_info_extended(struct repository *r,
 			     const struct object_id *,