[Feature] Update option: skip hashed files, add new files to hashfile

[simsong]

Converted from SourceForge issue 3484750, submitted by albert250

As I understand it, -X mode will recalculate hashes for ALL files, and display only the hashes of new (or changed) files. This means that if you have a directory with many huge files, and just add a couple of files to it, it will take ages to recalculate all hashes. I would like an "update" option to skip the hash checking of files existing in my md5 file, and only append the digests of new files.

The reason is that I don't expect existing files to change, but I want to keep a checksum file to be able to check copies to another disk. The directories I deal with are usually dozens of GB, so checking only the copy and not rechecking the original files makes a big difference.

Thanks a lot for these great utilities.