Improve version/commit loading to allow for some operations without loading the whole history

[npasserini]

See IceBranch>>addCommit

addCommit: newCommit
commits ifNotNil: [
"If commits are nil here, it means that they have not been loaded,
so the commit being added will be loaded when all commits are loaded.
Current strategy loads all commits at once, at some point we will require to be smarter,
and allow to work loading only the latest commit when it is not needed."
commits add: (self repository addCommit: newCommit)
]

[npasserini]

We will remove the history of commits in a branch.