[MERGE] Add CommitBuilder.abort()

[Ian Clatworthy]

Jelmer Vernooij wrote:
> This simple patch adds a CommitBuilder.abort() method for consistency.
> The only way to abort a commit without this function is to use the write
> group API, which is taken care of by CommitBuilder in all other code
> paths: it starts a write group when the CommitBuilder is created and
> commits a write group when the commit finishes.

bb: approve

Thanks for this. I ran across this yesterday and didn't like that except
clause much.

Ian C.