incremental verification for web special remote

incremental verification for web special remote

Except when configuration makes curl be used. It did not seem worth
trying to tail the file when curl is downloading.

But when an interrupted download is resumed, it does not read the whole
existing file to hash it. Same reason discussed in
commit 7eb3742e4b76d1d7a487c2c53bf25cda4ee5df43; that could take a long
time with no progress being displayed. And also there's an open http
request, which needs to be consumed; taking a long time to hash the file
might cause it to time out.

Also in passing implemented it for git and external special remotes when
downloading from the web. Several others like S3 are within striking
distance now as well.

Sponsored-by: Dartmouth College's DANDI project