split-index: accept that a base index can be empty

We are about to fix an ancient bug where `do_read_index()` pretended
that the index was not initialized when there are no index entries.

Before the `index_state` structure gained the `initialized` flag in
913e0e99b6 (unpack_trees(): protect the handcrafted in-core index from
read_cache(), 2008-08-23), that was the best we could do (even if it was
incorrect: it is totally possible to read a Git index file that contains
no index entries).

This pattern was repeated also in 998330ac2e (read-cache: look for
shared index files next to the index, too, 2021-08-26), which we fix
here by _not_ mistaking an empty base index for a missing
`sharedindex.*` file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -2500,12 +2500,14 @@ int read_index_from(struct index_state *istate, const char *path,
 
 	base_oid_hex = oid_to_hex(&split_index->base_oid);
 	base_path = xstrfmt("%s/sharedindex.%s", gitdir, base_oid_hex);
-	trace2_region_enter_printf("index", "shared/do_read_index",
-				   the_repository, "%s", base_path);
-	ret = do_read_index(split_index->base, base_path, 0);
-	trace2_region_leave_printf("index", "shared/do_read_index",
-				   the_repository, "%s", base_path);
-	if (!ret) {
+	if (file_exists(base_path)) {
+		trace2_region_enter_printf("index", "shared/do_read_index",
+					the_repository, "%s", base_path);
+
+		ret = do_read_index(split_index->base, base_path, 0);
+		trace2_region_leave_printf("index", "shared/do_read_index",
+					the_repository, "%s", base_path);
+	} else {
 		char *path_copy = xstrdup(path);
 		char *base_path2 = xstrfmt("%s/sharedindex.%s",
 					   dirname(path_copy), base_oid_hex);