[Featurev Req'] add either "--only-checksum" or "--ignore-size"

[zxzzz8]

please add one of these options, for the ability when you want it to only determine a change based on checksum only and ignore size and not just time,
and if the checksum is the same it will skip even if the size is reported to be different.

this is especially essential when backing up /sys for example,
as there are files there that are reported 4096 bits but are much less,
which cause many errors etc atm.

btw i would suggest when choosing what to implement or how,
to give people something that can let them freely pick any of the 3 types of checks like a toggle.
for example that there would also be a way to ignore size but not checksum AND time, or any other combination, like an on/off for each.
as there is already a request like #719 ,
and maybe even keeping an open mind like that for other new types of checks that might emerge in the future?
so that the ability of adding a new check in any combination of the others is easily implemented, by just adding another toggle for the new check as well.

thank you.