refactor(mcp): reduce complexity of find_merge_base in src/mcp.rs

[github-actions]

What was complex

find_merge_base in src/mcp.rs had a cognitive complexity of 18 (against a threshold of 10). The function handled three distinct concerns in a single deeply-nested body:

1. Candidate discovery — running git symbolic-ref refs/remotes/origin/HEAD and building the candidate list
2. Merge-base probing — looping through candidates with git merge-base, tracking whether any ref existed
3. Single-commit fallback — running git rev-list --max-parents=0 and --count HEAD with extra nesting to validate the "truly single-commit repo" condition

The four nested control-flow levels (loop → if-let → if-success → if-non-empty) plus the two-level fallback block pushed the complexity well above threshold.

What changed

Extracted three focused module-level async helper functions, placed in a new // Git merge-base helpers section:

Helper	Responsibility
build_merge_base_candidates	Runs git symbolic-ref, parses the result, appends common defaults
try_candidates_for_merge_base	Iterates candidates, tries git merge-base, tracks found_remote_ref
try_root_commit_fallback	Single-commit edge-case: checks root + total commit count, warns and returns SHA only when count ≤ 1

find_merge_base itself is now a four-line sequencing function:

async fn find_merge_base(git_dir: &std::path::Path) -> Result<String, McpError> {
    let candidates = build_merge_base_candidates(git_dir).await;
    let (base_sha, found_remote_ref) =
        try_candidates_for_merge_base(git_dir, &candidates).await;
    if let Some(sha) = base_sha { return Ok(sha); }
    if let Some(sha) = try_root_commit_fallback(git_dir).await { return Ok(sha); }
    Err(anyhow_to_mcp_error(if found_remote_ref { ... } else { ... }))
}

Before / After complexity

Function	Before	After
find_merge_base	18	~4 (no warning at threshold 10)
build_merge_base_candidates	—	~5
try_candidates_for_merge_base	—	~7
try_root_commit_fallback	—	~5

Verification

• All 2000+ tests pass (cargo test)
• cargo clippy --all-targets --all-features — clean, no new warnings
• find_merge_base no longer appears in the cognitive-complexity report at threshold 10
• No behaviour changes: all three git-command sequences, the found_remote_ref tracking, and both error messages are preserved verbatim in the extracted helpers

Warning

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