Consider implementing partial checksum

[imagejan]

Before deleting source files, we compare files by their os.stat() signature (filecmp.cmpfiles() with default shallow=True) here:

faim-robocopy/faim_robocopy/utils.py

Lines 52 to 54 in c03980f

	# and compare them (shallow).
	(matches, _, _) = filecmp.cmpfiles(current_dir, dest_dir,
	potential_matches)

This should be enough to also catch mismatches in creation dates (such as the 1980-01-01 ones created by Robocopy on incomplete files), but needs to be tested. We might want to offer an additional layer of safety.

Since comparing the full file contents (shallow=False) might decrease the performance too much, let's consider offering an option to compare partial checksum (e.g. on the first 128 kb) for each file, as suggested by @enricotagliavini.

/cc @lgelman