git repack won't delete any files until it finishes creating a new packfile. You might end up with a window in which the deletions are synced but not the upload of the new pack - this means git operations from other hosts may error out, but there'll be no permanent damage once the sync finishes. Additionally, git won't delete orphaned objects in automatic GCs until a certain time period passes - so no problem with concurrent commits. And if all else fails you can always undelete every file under .git/objects through dropbox :)