Cancel previous commit requests if selected ref changes

See this TODO comment: https://github.com/ForNeVeR/Fenrir/blob/f091f85b8b09cb1813cfbeb8bcbdf2870e46ad36/Fenrir/Ui/CommitsViewModel.fs#L36-L49

Consider this scenario:

1. User selects a ref named ref1
2. This code block starts asynchronous load of commits for ref1
3. User quickly changes their selection to a ref named ref2
4. This code block starts another asynchronous load
5. Load for ref2 my finish earlier than for ref1, so data for ref2 will be loaded into the commit control
6. And later, when the asynchronous load task for ref1 is finished, it will override the contents of the commit control, which is bad!