git: Use a buffer for the panel's commit message (#23308)

This PR changes the `GitPanel` and `GitState` to use a
`language::Buffer` for the commit message. This is a small initial step
toward remote editing and collaboration support.

Release Notes:

- N/A

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Co-authored-by: Max <max@zed.dev>
aa5fa4b7d4Cole Miller committed on 1/21/2025, 5:58:18 PM· committed by GitHubparent64f9acf
6 files changedLine totals unavailable