Recently, I ran into a problem doing a git review with just a changed commit message (no changed files). The error message was like:

# git review
error: unpack failed: error Missing tree 6f9d7b5d127584654beeb2cb0f160c21e521cf01
fatal: Unpack error, check server log
To ssh://

To solve this, push with the --no-thin option

# git push --no-thin gerrit HEAD:refs/publish/master
Counting objects: 5, done.
Delta compression using up to 2 threads.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (5/5), 3.42 KiB | 0 bytes/s, done.
Total 5 (delta 0), reused 0 (delta 0)
remote: Processing changes: updated: 1, refs: 1, done
remote: (W) e98f57f: no files changed, message updated