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.

Es werden keine Dateien oder Tests genannt. Beginne damit nachzuverfolgen, wie der Log-Tab Commits lädt und wie gitui die Konfiguration zugänglich macht, und ermittle dann, wo ein konfigurierbares oder fest codiertes Limit hingehört. Als erledigt gilt die Aufgabe, wenn große Repositorys nur die zulässigen neuesten Commits laden, ohne dass die Benutzeroberfläche hängen bleibt.

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