Idea: Improve hook speed by skipping before/after diff via readonly flag

https://github.com/pre-commit/pre-commit/blob/4f5cb99ff5c9e13edcd97461c09dcaf271197e5c/pre_commit/commands/run.py#L175-L185

Hi all,

I am looking at implemented pre-commit to replace a harder-to-maintain custom bash script our company current uses. In my initial tests, pre-commit works great, except for the fact that it is slow. The majority of our hooks are readonly in nature, and run quite fast. However, on our repository, pre-commit is spending close to 1 second per hook performing a before/after diff.

I added some instrumentation, and in our test case with 10 executed hooks, the execution time of pre-commit is >7 seconds with the before/after diffs, and ~1 second with those diffs disabled. I will note this is an extremely large repository, with many thousands of files. Generally, git actions are somewhat slow on it.

In exploring issues related to perf, I had found https://github.com/pre-commit/pre-commit/issues/510, which seems to tackle a different problem - individual hooks that take a while, rather than the overhead of pre-commit itself. However, there was one comment that got me thinking, regarding a readonly attribute one could apply to a hook. I know - this metadata would only be as truthy as the author who wrote the hook, but if a hook was marked as readonly, pre-commit could theoretically skip the before/after hook diffs.

Any thoughts? Happy to contribute the PR if maintainers see this as valuable.