Remove unneeded liftIO.
readProjectHead -> getProjectHead.
Kill special function for reading and saving HEADS file.
Fix readSnapshot type.
Factor out a function to get snapshot path by name.
saveSnapshot -> saveSnapshotFile

For uniformity with readSnapshotFile.
readSnapshot -> readSnapshotByName
Add saveSnasphotByName function.
Implement save command.
Don't resolve symbolic names when parsing reference.
Fix formatting.
A flag to allow overwriting existing snapshot.