### Please describe the problem.
Apparently, git-annex-lookupkey can't handle absolute paths to files to look for.

### What steps will reproduce the problem?

    /tmp % mkdir some
    /tmp % cd some
    /tmp/some % git init
    Initialized empty Git repository in /tmp/some/.git/
    /tmp/some % git annex init
    init  ok
    (recording state in git...)
    /tmp/some % echo some > some
    /tmp/some % git annex add some
    add some ok
    (recording state in git...)
    /tmp/some % git annex lookupkey some
    SHA256E-s5--2922bee6973370915cc63ab5ab8b7a57e1cab909477d7a030b2e4661e7aa2202
    /tmp/some % echo $?
    0
    /tmp/some % git annex lookupkey /tmp/some/some
    /tmp/some % echo $?
    1



### What version of git-annex are you using? On what operating system?
    % git annex version
    git-annex version: 6.20171018+gitgbb20b1ed3-1~ndall+1
    build flags: Assistant Webapp Pairing Testsuite S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify ConcurrentOutput TorrentParser MagicMime Feeds Quvi
    dependency versions: aws-0.14.1 bloomfilter-2.0.1.0 cryptonite-0.20 DAV-1.3.1 feed-0.3.11.1 ghc-8.0.1 http-client-0.4.31.1 persistent-sqlite-2.6 torrent-10000.0.0 uuid-1.3.12 yesod-1.4.3
    key/value backends: SHA256E SHA256 SHA512E SHA512 SHA224E SHA224 SHA384E SHA384 SHA3_256E SHA3_256 SHA3_512E SHA3_512 SHA3_224E SHA3_224 SHA3_384E SHA3_384 SKEIN256E SKEIN256 SKEIN512E SKEIN512 SHA1E SHA1 MD5E MD5 WORM URL
    remote types: git gcrypt p2p S3 bup directory rsync web bittorrent webdav tahoe glacier ddar hook external
    local repository version: 5
    supported repository versions: 3 5 6
    upgrade supported from repository versions: 0 1 2 3 4 5
    operating system: linux x86_64


### 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.
"""]]

### 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)

I do! I wouldn't even have my job, if it wasn't for git-annex. ;-)

> Which warms the cockles of my heart, Ben! :)
> 
> [[fixed|done]] --[[Joey]]
