### Please describe the problem.
I was accidentally syncing between repos at v5 and v6, I didn't realize they were different versions until I started digging into this issue.

I have two repos, one at v5 and one at v6. I add a file to v6 annex, unlock, then sync to my v5 repo. From my v5 repo, I then move the file into a sub-directory, in my v5 repo, and accidentally commit with git add; git commit, instead of git annex add; git commit, since with my v6 repo I am used to using git add for everything. The file content is now missing from annex on my v5 repo. I then sync from both repos. Now the file in v5 repo is broken and the content is not present in annex. The content is fine on the v6 repo. The only way to actually get annex to see the content, from my v5 repo, though is to do git annex upgrade, git annex sync, fsck repair don't help.

### What steps will reproduce the problem?

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

    $ git annex version
    git-annex version: 6.20171128-g58b04cd2e
    build flags: Assistant Webapp Pairing Testsuite S3(multipartupload)(storageclasses) WebDAV FsEvents ConcurrentOutput TorrentParser MagicMime Feeds Quvi
    dependency versions: aws-0.17.1 bloomfilter-2.0.1.0 cryptonite-0.23 DAV-1.3.1 feed-0.3.12.0 ghc-8.0.2 http-client-0.5.7.0 persistent-sqlite-2.6.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.4.5
    key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 SHA1E SHA1         MD5E MD5 WORM URL
    remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
    local repository version: 6
    supported repository versions: 3 5 6
    upgrade supported from repository versions: 0 1 2 3 4 5
    operating system: darwin x86_64

### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.

#
# Create a V6 repo and add an image to it
#
$ mkdir /tmp/repov6
$ cd /tmp/repov6
$ git init
$ git annex init
$ /tmp/repov6$ git annex upgrade
upgrade . (v5 to v6...) ok

$ git add image1.png 
$ git commit -m "added image 1 to annex"
$ /tmp/repov6$ git annex info image1.png 
file: image1.png
size: 17.69 kilobytes
key: SHA256E-s17691--c98e6eae515fec6a094e2b4bdcd221ba089a5e6073f6c365f6d4c829243c8aa5.png
present: true
$ git annex unlock image1.png
$ git commit -m "unlocked the image"

#
# Create a V5 repo
# and sync with first repo
#
$ cd /tmp
$ git clone /tmp/repov6/ repov5
Cloning into 'repov5'...
done.
$ cd repov5
$ git annex init "repo v5"
$ git remote add repov6 /tmp/repov6
$ git annex sync
andrew@bumblebee /tmp/repov5$ ls -l image1.png 
… image1.png -> .git/annex/objects/zJ/7J/SHA256E-s17691--c98e6eae515fec6a094e2b4bdcd221ba089a5e6073f6c365f6d4c829243c8aa5.png/SHA256E-s17691--c98e6eae515fec6a094e2b4bdcd221ba089a5e6073f6c365f6d4c829243c8aa5.png

# this repo is still at version 5
$ git annex version
…
local repository version: 5
…

# now lets move the unlocked not present image file
$ mkdir imagesFolder
$ mv image1.png imagesFolder/
$ git add -A .
$ git commit -m "moved file"

## oops I accidentally committed my annexed file using git add from a v5 repo…
## since I am used to using git add commands in my v6 repos

#
# Now lets sync
#
$ cd /tmp/repov6
$ git remote add repov5 /tmp/repov5
$ git annex sync
$ ls -laL imagesFolder/image1.png 
…  17691 Dec 10 21:26 imagesFolder/image1.png

# 
# Now lets try to get this to our V5 repo
#
$ cd /tmp/repov5
$ git annex sync
$ git annex info imagesFolder2/image1.png
fatal: Not a valid object name imagesFolder2/image1.png
info imagesFolder2/image1.png (not a directory or an annexed file or a treeish or a remote or a uuid) failed
a

# the following does not help
$ git annex fsck
$ git annex repair

# the following resolves the issue
$ git annex upgrade	


# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log


# End of transcript or log.
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)
Yes

> I don't consider this a bug, so [[done]] --[[Joey]]
