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)
