### Please describe the problem.
Cannot lock files which are not present.  This prevents converting v6 files from flat files to symlinks when they are missing.
If a file is not present, locking it should either replace it with a broken symlink, or continue on to the next file with a warning.

### What steps will reproduce the problem?
I think this happened from unlocking a file in one v6 repo and syncing with a different v6 repo, perhaps via a v5 repo in the middle.  In repo #2, the file became unlocked but missing for me.

### What version of git-annex are you using? On what operating system?
6.20160528-g191d2ba on linux

### Please provide any additional information below.

[[!format sh """

# I am trying to lock all files in my repo to downgrade from v6 back to v5, for stability

$ git annex lock
lock git/bup.git/bupindex.hlink git-annex: content not present; cannot lock

# but I cannot
"""]]

### 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)

> Well, that was simply not implemented, but I've done so now. (unlocking
> too). [[done]] --[[Joey]]
