### Please describe the problem.

Accidentally leaving of the file name when invoking git annex metadata means that 
the changes are applied to all files. This is almost never what I want, and generally leads to either manual work or trying to undo the commit on the git-annex branch.

### What steps will reproduce the problem?

% git annex metadata -s field=value

congrats, your old values of field are (kinda) gone.

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

5.20141125, debian

### 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


# End of transcript or log.
"""]]

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