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!