improve performance of diffs (#32393)

This has two major changes that significantly reduce the amount of work
done for large diffs:

* Kill a running git process when reaching the maximum number of files
in a diff, preventing it from processing the entire diff.
* When loading a diff with the URL param `file-only=true`, skip loading
stats. This speeds up loading both hidden files of a diff and sections
of a diff when clicking the "Show More" button.

A couple of minor things from profiling are also included:

* Reuse existing repo in `PrepareViewPullInfo` if head and base are the
same.

The performance impact is going to depend heavily on the individual diff
and the hardware it runs on, but when testing locally on a diff changing
100k+ lines over hundreds of files, I'm seeing a roughly 75% reduction
in time to load the result of "Show More"

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

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