infinitepush: ensure fileindex bookmarks use '/' separators (issue5840) After loading up with status messages, I noticed that the subsequent matcher was rejecting 'scratch\mybranch' on Windows. No bookmarks were reported back, and the tests subsequently failed. I did a search for 'match', and nothing else looks like it needs to be fixed up, but someone who understands this code should also take a look. I also tried setting `infinitepush.branchpattern=re:scratch\\.*` in library-infinitepush.sh without this change, but that didn't work. Still, should we ban '\' in these bookmarks to avoid confusion? I thought I saw code that sandwiches a pattern between 're:^' and '.*', so perhaps regex characters will need special care? I also noticed comments in externalbundlestore.{read,write} that it won't work on Windows because of opening an open file. But I don't see a test failure, so this may lack test coverage.