Make fast blob reads optional

[andrew]

Moves the in-process go-git blob reader into the nested github.com/git-pkgs/clone/gogit module. The root Blob and InspectBlob functions use git show, so clone-only consumers no longer inherit the go-git module graph. Scrutineer can opt into gogit and keep the fast reader. Includes tests, CI coverage, Dependabot configuration, and usage documentation for both modules.

Closes #7.

[Copilot]

Pull request overview

This PR makes the fast in-process blob reader optional by moving the go-git implementation into a nested github.com/git-pkgs/clone/gogit module, while keeping the root clone module’s blob reads implemented via git show to reduce the dependency graph for clone-only consumers.

Changes:

• Refactors root clone.Blob / clone.InspectBlob to read blobs via the git binary (removing go-git dependencies from the root module graph).
• Adds a nested gogit module containing the go-git-backed blob reader, with tests/benchmarks and compatibility fallback to git show.
• Updates documentation and CI/Dependabot to account for the additional module.

Reviewed changes

Copilot reviewed 13 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
README.md	Documents root blob behavior (git show) and the opt-in gogit module usage.
doc.go	Updates package docs to reference the optional gogit module for in-process blob reads.
go.mod	Removes go-git dependencies from the root module.
go.sum	Drops go-git-related sums from the root module.
blob.go	Switches root blob reads to git show only.
blob_test.go	Updates tests to reflect git being required on PATH for root blob reads; moves go-git coverage out.
gogit/doc.go	Adds package docs for the gogit module.
gogit/blob.go	Implements go-git-backed Blob/InspectBlob with fallback to root clone git-based behavior.
gogit/blob_test.go	Adds tests/benchmarks for the gogit blob reader across repo layouts and fallbacks.
gogit/object_store.go	Moves/renames the go-git object-store implementation into package gogit.
gogit/test_helpers_test.go	Adds git test helpers and deterministic git environment for gogit tests.
gogit/go.mod	Introduces the nested module definition and dependencies (including clone).
gogit/go.sum	Adds sums for the nested module’s dependency set (go-git graph).
.github/workflows/ci.yml	Builds/tests/lints both the root module and the gogit submodule in CI.
.github/dependabot.yml	Adds Dependabot updates for gomod in /gogit.

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