do_read_index(): always mark as initialized unless erroring out

Even when `.git/index` did not exist (and that was indicated as being
okay), we need to mark the index as initialized when returning from
`do_read_index()` with success.

This will become relevant in the next commit, where an incorrect
condition trying to determine whether an in-memory index was initialized
(using `cache_nr == 0` as an indicator, which is wrong) will be replaced
with the correct condition (`initialized != 0`).

Signed-off-by: Johannes Schindelin <[email protected]>

read-cache.c

@@ -2285,6 +2285,7 @@ int do_read_index(struct index_state *istate, const char *path, int must_exist)
 	if (fd < 0) {
 		if (!must_exist && errno == ENOENT) {
 			set_new_index_sparsity(istate);
+			istate->initialized = 1;
 			return 0;
 		}
 		die_errno(_("%s: index file open failed"), path);