git_ui: Add support for generating commit messages with an LLM (#26227)

This PR finishes up the support for generating commit messages using an
LLM.

We're shelling out to `git diff` to get the diff text, as it seemed more
efficient than attempting to reconstruct the diff ourselves from our
internal Git state.


https://github.com/user-attachments/assets/9bcf30a7-7a08-4f49-a753-72a5d954bddd

Release Notes:

- Git Beta: Added support for generating commit messages using a
language model.

---------

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

assets/keymaps/default-linux.json

@@ -747,7 +747,8 @@
     "context": "GitCommit > Editor",
     "bindings": {
       "enter": "editor::Newline",
-      "ctrl-enter": "git::Commit"
+      "ctrl-enter": "git::Commit",
+      "alt-l": "git::GenerateCommitMessage"
     }
   },
   {
@@ -769,7 +770,8 @@
       "tab": "git_panel::FocusChanges",
       "shift-tab": "git_panel::FocusChanges",
       "ctrl-enter": "git::Commit",
-      "alt-up": "git_panel::FocusChanges"
+      "alt-up": "git_panel::FocusChanges",
+      "alt-l": "git::GenerateCommitMessage"
     }
   },
   {