### Please describe the problem.
git-annex-shell -c git-annex-shell errors, and git-annex interprets this as meaning that git-annex-shell is normal git-shell.

### What steps will reproduce the problem?
Running git-annex-shell -c git-annex-shell.

### What version of git-annex are you using? On what operating system?
NixOS, 6.20180529.

### 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

[leo60228@nixos:~/test]$ git-annex-shell -c git-annex-shell
fatal: unrecognized command 'git-annex-shell'
git-annex-shell: git-shell failed

# 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.

[[moreinfo]]
