Skip writing unchanged generated files to preserve mtimes

[iainmcgin]

Contributes towards #3126.

Summary

buf generate previously wrote output files unconditionally, touching mtimes even when content was identical to what was already on disk. For mtime-based build systems (cargo, make, ninja, file-watching dev servers), a no-op buf generate would cascade into unnecessary downstream rebuilds.

The protoc plugin protocol gives plugins no visibility into the output directory—they emit File{name, content} on stdout and the invoking tool handles filesystem writes—so this fix lives in buf's write path rather than in individual plugins.

Approach

Replaces the unconditional storage.Copy in the directory output path (bufprotopluginos.writeDirectory) with a compare-then-write loop. For each generated file:

• Read existing content from the OS bucket
• If it matches the new content, skip the write (mtime preserved)
• Otherwise write as before

Read errors during comparison (file doesn't exist, permissions, etc.) fall through to write, so worst-case behavior is identical to before.

Scope

• Directory output only. zip/jar output continues to write unconditionally—comparing archive content would require unzip-compare-rezip, which isn't motivated by the use cases in Make buf generate --clean a per-plugin option that does a "smart" clean #3126 and can be a follow-up if needed.
• Default-on, no flag. Per the discussion in Make buf generate --clean a per-plugin option that does a "smart" clean #3126, this is backwards-compatible: it's never wrong to skip writing identical content, and the cost is one extra read per existing file (zero cost on first run).
• --clean interaction is already correct. Clean deletes first, so comparison finds nothing and writes everything.

Testing

• 4 unit tests in response_writer_test.go covering unchanged/changed/new/mixed file scenarios
• 1 integration test in generate_test.go that runs buf generate twice and verifies the output mtime is preserved

[emcfarlane]

Thanks for this contribution! The skip-unchanged behavior is a good improvement.

However, this doesn't fully close #3126. That issue is about making --clean a per-plugin "smart" clean, including stale file deletion. This PR is a useful step toward that, but the core of the issue remains open.

Could you update the PR description to say "Contributes to #3126" instead of "Closes #3126" so the issue stays open?

[emcfarlane]

Suggested change

	- `buf generate` now skips writing output files when the content matches what's already on disk, preserving modification times for mtime-based build systems like cargo and make.
	- Skip writing unchanged output files in `buf generate` to preserve modification times

[doriable]

Looks good to me, thanks for putting this up! +1 to Ed's comments on fixing the PR description.

[iainmcgin]

Changed to "contributes towards" as requested