Fix plotting of initial data in plot_fluxes

`final['inputFlux'] - final['fittedFlux']` is comparing "initial inputFlux"
with "final fittedFlux", which will give much larger residuals than
`initial['inputFlux'] - initial['fittedFlux']`, because fittedFlux changes
during the fit. The error was most noticeable on HSC data. The per-visit part
of the photometric calibration is a few percent correction at most; the
difference between the initial and final plots should be relatively minor.