Include file contents when generating file hash

[timkpaine]

Description

Include file contents when generating file hash, in case different files have the same contents.

Fixes: https://github.com/conda/conda-build/issues/4814 Xref: https://github.com/conda-forge/staged-recipes/pull/22323

Checklist - did you ...

• [ ] Add a file to the news directory (using the template) for the next release's release notes?
• [x] Add / update necessary tests?

I'm not sure how / where to test this, please advise.

• [x] Add / update outdated documentation?

[conda-bot]

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[jezdez]

@conda/builds-tools Any suggestion where @timkpaine could add a test for this?

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #4821 will not alter performance

_{Comparing timkpaine:tkp/4814 (312c888) with main (01b47f4)}

Summary

✅ 3 untouched benchmarks

[isuruf]

pre-commit.ci autofix

[isuruf]

Ready for review @conda/builds-tools