git: Ensure no more than 4 blame processes run concurrently for each multibuffer (#44843)

Previously we were only awaiting on up to 4 of the blame futures at a
time, but we would still call `Project::blame_buffer` eagerly for every
buffer in the multibuffer. Since that returns a `Task`, all the blame
invocations were still launched concurrently.

Release Notes:

- N/A
7a4de734c6Cole Miller committed on 12/15/2025, 11:50:18 PM· committed by GitHubparentb8d0da9
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