When running `sync-files` (or `sync-identical-files`) with the `--latest` switch, if one or more of the files in a group does not exist, the script will crash. This happens all the time when I add a new group, or add a new file path in an existing group. This has bothered me for a long time, so I finally fixed it when I ran into it again today.
I've changed the script as follows:
- If _none_ of the paths in the group exist, print an error message listing the paths in the group. This happens with or without `--latest`.
- If `--latest` is specified, copy the master file to the paths of the missing files.
We currently use a script to keep certain duplicate QL files in sync across the repo. For historical reasons, this script has lived in the private repo alongside the rest of CodeQL, even though it's only used for files in the public `ql` repo. This PR moves the script into the public `ql` repo. It is still invoked by Jenkins scripts that live in the private repo during CI, but it can also be invoked directly without having a checkout of the private repo. This is useful for anyone who is modifying the dataflow or IR libraries with only a QL checkout.
