### Please describe the problem.

someone kinda could consider it a 'feature' but it complicates use of the output since then progressbar should jump down and some progressbar libraries 
do not "support" that

### What steps will reproduce the problem?

initiate download, interrupt it, try to redownload it... I guess in some cases redownload doesn't start at the point where it was previously interrupted but somewhat before, or restarts altogether.  But annex first reports in --json-progress the size of previously downloaded portion and then goes down.  see below

so, my life would be easier, if annex did not report "unconfirmed initial progress" at all I guess

### What version of git-annex are you using? On what operating system?

6.20170307+gitg24ade8a25-1~ndall+1

### Please provide any additional information below.

[[!format sh """
$> ls -l .git/annex/tmp; k=$(/bin/ls .git/annex/tmp | head -n 1); [ -z "$k" ] || git annex get --json --json-progress --key $k
total 3796
-rw------- 1 yoh yoh 3887104 Mar 24 17:06 MD5E-s4108657--e055fc250b37b313c0904f3687bbed1c
{"byte-progress":3887104,"action":{"command":"get","note":"from origin...","key":"MD5E-s4108657--e055fc250b37b313c0904f3687bbed1c","file":null},"total-size":4108657,"percent-progress":"94.61%"}
{"byte-progress":2068480,"action":{"command":"get","note":"from origin...","key":"MD5E-s4108657--e055fc250b37b313c0904f3687bbed1c","file":null},"total-size":4108657,"percent-progress":"50.34%"}
{"command":"get","note":"checksum...","success":true,"key":"MD5E-s4108657--e055fc250b37b313c0904f3687bbed1c","file":null}

"""]]


