### Please describe the problem.
I'm playing around with the new export function and it's working really fine so far. But just now I ran into problems when `git add`ing a file, see below. 

### What steps will reproduce the problem?

[[!format sh """

$ echo test > file.txt
$ git add file.txt
$ git annex sync
commit  
[master a05faa1] git-annex in lykos@**:/tmp/testrepo2
 1 file changed, 1 insertion(+)
 create mode 100644 file.txt
ok
$ git annex export --tracking master --to exp
export exp file.txt 
git-annex: Sorry, this file cannot be stored on an external special remote because its key's name contains a space. To avoid this problem, you can run: git-annex migrate --backend=SHA1 and pass it the name of the file
failed
(recording state in git...)
git-annex: export: 1 failed

"""]]


### What version of git-annex are you using? On what operating system?
6.20171026-g43d011a52 on Arch Linux

### 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)
Of course ;) All the time

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