Add configurable upper limit to number of commits loaded in Log tab

**Is your feature request related to a problem? Please describe.**
Opening the "Log [2]" tab in a repo with an extreme number of commits (3 million in my case) continues to load all commits well beyond anything I would reasonably need to navigate to. This also causes the interface to hang and run very slowly.

**Describe the solution you'd like**
I would like there to be a configurable upper limit to the number of commits gitui loads in the "Log" tab. A hard-coded upper limit somewhere between 10-100k would also suffice. This would allow me to only load the most recent commits and keep the UI very fluid. For my workflows, the code that was modified 100k+ commits ago is not something I'm likely to ever need to look at from the "Log [2]" tab.

**Describe alternatives you've considered**
As it stands now, I'd love to use gitui in this large repo, but the response time in this gets too slow to use it. Even tabbing through the log on accident causes the interface to hang. I end up using the terminal based git commands most of the time instead.

ファイルやテストは指定されていません。まず、Log タブがどのようにコミットを読み込み、gitui がどのように設定を公開しているかを追跡し、次に設定可能な制限またはハードコードされた制限をどこに置くべきかを判断してください。大規模なリポジトリが許可された最近のコミットのみを読み込み、インターフェースがハングしなければ完了です。

索引モデルが issue の本文から書いたものです。