make s3_put and gd_put smarter?

S3_put and gd_put could check the file hashes before sending files over the wire. This may be redundant with what remake already knows from indicator files though.

Start by locating the R entry points for s3_put and gd_put and reviewing how remake indicator files currently track file state. Determine the intended hash comparison and transfer-skipping behavior before changing anything; done should mean matching files are not sent unnecessarily and both upload paths have verified behavior.

Written by the indexing model from the issue text.