Merge branch 'tk/untracked-cache-with-uall' into maint

Fix for a bug that makes write-tree to fail to write out a
non-existent index as a tree, introduced in 2.37.
source: <20220722212232.833188-1-martin.agren@gmail.com>

* tk/untracked-cache-with-uall:
  read-cache: make `do_read_index()` always set up `istate->repo`

read-cache.c

@@ -2294,6 +2294,8 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	fd = open(path, O_RDONLY);
 	if (fd < 0) {
 		if (!must_exist && errno == ENOENT) {
+			if (!istate->repo)
+				istate->repo = the_repository;
 			set_new_index_sparsity(istate);
 			return 0;
 		}

t/t7063-status-untracked-cache.sh

@@ -985,4 +985,9 @@ test_expect_success '"status" after file replacement should be clean with UC=fal
 	status_is_clean
 '
 
+test_expect_success 'empty repo (no index) and core.untrackedCache' '
+	git init emptyrepo &&
+	git -C emptyrepo -c core.untrackedCache=true write-tree
+'
+
 test_done