Clear it in 3 steps
Start the environment
Investigate the target
Submit the flag

/app/repo is a Git repository. A developer accidentally deleted a commit containing important information. Use git reflog to find the deleted commit and retrieve the flag in its commit message. Hint: git reflog shows the history of operations
Check HEAD movement history with git reflog
Once you find the deleted commit hash, use git show <hash>
The commit message contains "FLAG:"