Memory leak when there are too many changes in the repo

[brucehsu]

Currently the Git pane tries to load every changes and render them in one go, which in extreme cases (for example big merges), will attempt to allocate all the memory it could and hang the system.

The propose solution is to do what GitHub is doing for large PRs: only proactively load the first few changes, and leave the rest for explicit demand.

Another improvement for responsiveness is make the change loading asynchronous i.e. not blocking the main thread.