receive-pack: fix "borrowing from alternate object store" implementation

In the alternate_object_database structure, ent->base[] is a buffer the
users can use to form pathnames to loose objects, and ent->name is a
pointer into that buffer (it points at one beyond ".git/objects/").  If
you get a call to add_refs_from_alternate() after somebody used the entry
(has_loose_object() has been called, for example), *ent->name would not be
NUL, and ent->base[] won't be the path to the object store.

This caller is expecting to read the path to the object store in ent->base[];
it needs to NUL terminate the buffer if it wants to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-receive-pack.c

@@ -466,12 +466,17 @@ static int delete_only(struct command *cmd)
 
 static int add_refs_from_alternate(struct alternate_object_database *e, void *unused)
 {
-	char *other = xstrdup(make_absolute_path(e->base));
-	size_t len = strlen(other);
+	char *other;
+	size_t len;
 	struct remote *remote;
 	struct transport *transport;
 	const struct ref *extra;
 
+	e->name[-1] = '\0';
+	other = xstrdup(make_absolute_path(e->base));
+	e->name[-1] = '/';
+	len = strlen(other);
+
 	while (other[len-1] == '/')
 		other[--len] = '\0';
 	if (len < 8 || memcmp(other + len - 8, "/objects", 8))