Lots of memory usage with GBs sized files in diff

**Describe the bug**

Opening the app in a repo containing a large uncommitted file has gitui quickly consuming all available memory on my system. I have to quit quickly before my system halts.

**To Reproduce**

1. place a multi GB file in a repo
2. open `gitui`
3. scroll down and select this file
4. (optional) scroll away and select another file, notice that memory usage continues to climb

**Expected behaviour**

It took me a while to notice the issue, as it was taking down my machine before I could play-around, I was assuming it was another cause.

It appears that gitui was trying to present the diff of the new uncommitted file to me. I believe the flame graph below confirms that `git_diff_file_content__init_from_src()`.

Some potential solutions:

1. Perhaps have a limit on the size of file contents we're willing to present to the user in the diff viewer
2. or perhaps only present the file content on-focus of the diff viewer (on the right)

**Screenshots**

**Context (please complete the following information):**
- Fedora Linux 38.20230520.0 (Silverblue)
- gitui 0.20.1
- Rust version: 1.69.0

**Additional context**

`make profile` which produced the following flamegraph:


Beginne damit, den Bericht mit einer nicht committeten Datei von mehreren GB in gitui zu reproduzieren, und verwende dann den gemeldeten Pfad `git_diff_file_content__init_from_src()` sowie `make profile`, um das Speicherwachstum beim Auswählen und Wechseln von Dateien zu untersuchen. Als erledigt gilt die Aufgabe, wenn der Diff-Viewer für große Dateien keinen unbegrenzten Speicher mehr verbraucht, während normale Datei-Diffs weiterhin nutzbar bleiben.

Vom Indexierungsmodell aus dem Issue-Text verfasst.