feat(buffetch): add merge_base option to buf breaking --against

[jasondamour]

Closes #4495

Summary

Adds a merge_base=<ref> option to the --against flag of buf breaking, so users can write:

buf breaking --against ".git#merge_base=main"

instead of the shell workaround:

buf breaking --against ".git#ref=$(git merge-base HEAD main)"

This is especially useful in pre-commit hook tools (e.g. prek) that don't expose the merge-base commit as an environment variable.

How it works

When merge_base=<ref> is specified:

1. The option is parsed through the existing buffetch ref-parsing pipeline and stored on the GitRef interface as GitMergeBase() string.
2. At clone time in getGitBucket(), buf runs git merge-base HEAD <ref> against the local repository to resolve the common ancestor commit hash.
3. That commit hash is used as the checkout target (equivalent to ref=<hash>), so the comparison is against the exact point where the current branch diverged from <ref>.

merge_base is only supported for local git references (e.g. .git#merge_base=main). For remote repos, the existing shell workaround ref=$(git merge-base HEAD origin/main) remains the approach.

Constraints

• Cannot be combined with branch, commit, tag, or ref (returns a clear error)
• Defaults to depth=50 (same as ref=)
• Only valid for local git schemes

Changes

File	Change
private/pkg/git/git.go	Add GetMergeBase() helper
private/buf/buffetch/internal/errors.go	Add 2 error constructors
private/buf/buffetch/internal/internal.go	Add GitMergeBase to RawRef and GitRef interface
private/buf/buffetch/internal/git_ref.go	Add gitMergeBase field and accessor
private/buf/buffetch/internal/ref_parser.go	Parse, validate, and thread merge_base through
private/buf/buffetch/internal/reader.go	Resolve merge-base commit before cloning
private/buf/buffetch/internal/ref_parser_test.go	Unit tests for parsing and validation

Test plan

• Unit tests added in ref_parser_test.go covering valid parsing and all invalid combinations
• buf breaking . --against ".git#merge_base=main" produces identical output to buf breaking . --against ".git#ref=$(git merge-base HEAD main)"
• All existing buffetch and git package tests pass

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