Git "You have not concluded your merge" and nothing to commit?

Did you end up with an empty merge commit (two parents) or just an empty commit? In the latter case, you could have deleted .git/MERGE_HEAD.

Update: Rather than delete MERGE_HEAD by hand, you could also use git merge --abort (as of git 1.7.4) or git reset --merge (as of git 1.6.2).

It's also worth mentioning that at least as of git 1.8.3 (maybe earlier?) you should see a status message like this if an actual merge is in progress and needs to be committed (if you specified --no-commit, for example):

# On branch master
# All conflicts fixed but you are still merging.
#   (use "git commit" to conclude merge)
#
nothing to commit, working directory clean

If you don't see this and still get the MERGE_HEAD warning, something is messed up and you should probably just --abort to get back to a clean state.

Additional Detail from Comments

During a merge, git creates a MERGE_HEAD file in the root of the .git folder (next to HEAD, ORIG_HEAD, probably FETCH_HEAD, etc) to track information about the merge in progress (specifically, the SHA(s) of the commit(s) being merged into the current HEAD). If you delete that, Git will no longer think a merge is in progress. Obviously, if a merge really is in progress then you wouldn't want to delete this file.

Okay I finally found answer: git commit -m "Test" apparently fixed this. The result was an empty commit with no changes whatsoever. Even Github shows an empty commit, but it works.