Build a mini version control system like git

• init() creates an empty repository with a main branch; a working directory is a map of file paths to contents that the caller modifies through writeFile and deleteFile
• add(paths) stages files, and commit(message, author) stores a snapshot: each file's content is stored as a blob under the SHA-1 or SHA-256 hash of its content (identical content is stored once), and a commit records the tree (path to blob hash), parent hash and message
• A commit's id is the hash of its own contents (tree, parent, message, author, timestamp), so changing history changes the id
• log() lists commits from the current head backwards, following parents; status() shows staged, modified and untracked files compared with the last commit
• branch(name) creates a branch pointing at the current commit, and checkout(name) switches to it and restores the working directory from that commit, refusing when there are uncommitted changes that would be lost
• diff(commitA, commitB) (or working copy versus head) lists added, deleted and modified files, and for modified text files shows line changes using a longest common subsequence diff

Only after every requirement works.

• Merge of two branches with fast-forward detection and conflict markers
• reset and revert
• Garbage collection of unreachable objects