Add content hash support + support EARTH_GIT_* builtins

[gilescope]

Adds EARTH_GIT_CONTENT_HASH to surface git's HEAD^{tree} stable content hash.
This doesn't change with squashs, merges or rebases - as long as the git contents is the same the hash is the same.

While we're in the area, ensures all the existing EARTHLY_GIT_* can be used as EARTH_GIT_* also and docs refer to those now.

Fixes #339

[github-actions]

🎉 Are we earthbuild yet?

Great progress! You've reduced "earthly" occurrences by 11 (0.20%)

📈 Overall Progress

Branch	Total Count
main	5626
This PR	5615
Difference	-11 (0.2%)

📁 Changes by file type:

File Type	Change
Go files (.go)	➖ No change
Documentation (.md)	✅ -11
Earthfiles	➖ No change

---

Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

Run locally to see detailed breakdown:

./.github/scripts/count-earthly.sh

Note that the goal is not to reach 0.
There is anticipated to be at least some occurences of earthly in the source code due to backwards compatibility with config files and language constructs.

[gemini-code-assist]

Summary of Changes

Hello @gilescope, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the build system by introducing support for Git's content hash, also known as the tree hash. This new capability allows users to access a stable, content-addressable identifier for the current state of the Git repository's working tree, which is crucial for ensuring build reproducibility and caching. The content hash is now exposed via the EARTHBUILD_GIT_CONTENT_HASH variable.

Highlights

• Git Content Hash Support: Introduced support for Git content hash (tree hash) to be exposed via the EARTHBUILD_GIT_CONTENT_HASH variable.
• Data Structure Enhancements: Added a contentHash field to resolvedGitProject and GitMetadata structs to store the Git tree hash.
• Git Command Integration: Implemented logic to retrieve the Git tree hash using the git rev-parse HEAD^{tree} command.
• Built-in Variable Exposure: Included EARTHBUILD_GIT_CONTENT_HASH as a new built-in argument, making the content hash accessible within the build system.

Changelog

• buildcontext/git.go
  • Added contentHash field to resolvedGitProject struct.
  • Populated contentHash by executing git rev-parse HEAD^{tree}.
  • Passed the contentHash to the resolveEarthProject function.
• util/gitutil/detectgit.go
  • Defined ErrCouldNotDetectGitContentHash for error handling.
  • Added ContentHash field to GitMetadata struct.
  • Implemented detectGitContentHash function to retrieve the Git tree hash.
  • Integrated detectGitContentHash into the Metadata function.
• util/gitutil/detectgit_test.go
  • Added TestDetectGitContentHash to verify the correct detection of the Git content hash, including scenarios with amended commits.
• variables/builtin.go
  • Added EarthbuildGitContentHash to the BuiltinArgs function, making the content hash available as a built-in variable.
• variables/builtin_test.go
  • Added TestBuiltinArgsContentHash to confirm the content hash is correctly added to the scope.
  • Added TestBuiltinArgsContentHashNilGitMeta to handle cases where Git metadata is not available.
• variables/reserved/names.go
  • Declared EarthbuildGitContentHash as a new constant.
  • Registered EarthbuildGitContentHash in the args map of reserved variable names.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request effectively adds support for EARTHBUILD_GIT_CONTENT_HASH. The implementation is solid, and the accompanying tests are thorough and well-designed. My feedback focuses on improving maintainability by reducing code duplication in two areas by abstracting repeated logic into helper functions.

[kmannislands]

Very interesting. Definitely see how this could be useful.

Only feedback is one naming

[gilescope]

Refactored the code to remove the duplication and switched prefix. Potentially this PR is ready.