### Please describe the problem.
A bare v7 repo fails to upgrade to v8, and displays several times the following error message: "fatal: relative path syntax can't be used outside working tree"


### What steps will reproduce the problem?
```
cd problem_repo.git
git annex upgrade
```

### What version of git-annex are you using? On what operating system?
git-annex version: 8.20200617

### Please provide any additional information below.

[[!format sh """
# If you can, paste a complete transcript of the problem occurring here.
# If the problem is with the git-annex assistant, paste in .git/annex/daemon.log

upgrade (v7 to v8...) fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree
fatal: relative path syntax can't be used outside working tree

git-annex: fd:18: Data.ByteString.hGetLine: end of file
failed
git-annex: user error (git ["--git-dir=.","--literal-pathspecs","cat-file","--batch-check=%(objectname) %(objecttype) %(objectsize)"] exited 128)


# 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! We love it! Thank you so much! Git-annex is a superpower 🦹‍♂️ 

> [[fixed|done]] --[[Joey]]
