The commit mode ensures pictures are not corrupted when Instaloader is
unexpectedly interrupted. In the case that the last picture is corrupted
because of an interruption, Instaloader will redownload the picture.
Since the metadata is the last object saved to disk, we can consider a
post as "committed" if its json metadata file exists and is not
malformed. Instaloader should download any posts which are not
committed. Downside is commit mode requires metadata to be saved.
sushilicious