Performance marks are broken when in File-by-File mode

When loading diffs, performance marks are started when the load begins and ended by individual files (e.g. the last file in the list ends the measurement).

This is broken in file-by-file mode for two reasons:

1. The Vuex action that starts the performance measurement is never used because a new action for loading a single file is called instead.
2. The concept of "first" or "last" files is nonsense when loading a single file at a time, as every file is the first file, the last file, and all files.