configurable "file was changed while we backed it up"

Currently, borg uses the `ctime` timestamp to detect whether a file was changed while it was being backed up.

There are filesystems where this does not work, e.g. the fs implemented by OneDrive on macOS. See #8676.

So, using ctime could be the default and mtime could be requested by a cli option like `--file-changed=mtime`.