phase 3 initial completion

internal/domain/git/binary.go

@@ -138,3 +138,67 @@ func TreeLS(repoPath, ref, subPath string) ([]TreeEntry, error) {
 func BlobCat(repoPath, ref, filePath string) ([]byte, error) {
 	return run(repoPath, "show", ref+":"+filePath)
 }
+
+// FileDiff represents a single changed file in a diff.
+type FileDiff struct {
+	Path      string `json:"path"`
+	OldPath   string `json:"oldPath,omitempty"` // set on renames
+	Additions int    `json:"additions"`
+	Deletions int    `json:"deletions"`
+	Patch     string `json:"patch"` // unified diff hunk(s) for this file
+}
+
+// Diff returns per-file diffs between two refs.
+func Diff(repoPath, base, head string) ([]FileDiff, error) {
+	out, err := run(repoPath,
+		"diff", "--unified=5", "--no-color",
+		"--diff-filter=ACDMRT", // exclude untracked
+		base+".."+head,
+	)
+	if err != nil {
+		return nil, err
+	}
+	return parseUnifiedDiff(string(out)), nil
+}
+
+// parseUnifiedDiff splits a multi-file unified diff into per-file FileDiff entries.
+func parseUnifiedDiff(raw string) []FileDiff {
+	var files []FileDiff
+	var cur *FileDiff
+	var patchLines []string
+
+	commit := func() {
+		if cur != nil {
+			cur.Patch = strings.Join(patchLines, "\n")
+			files = append(files, *cur)
+		}
+	}
+
+	for _, line := range strings.Split(raw, "\n") {
+		if strings.HasPrefix(line, "diff --git ") {
+			commit()
+			cur = &FileDiff{}
+			patchLines = nil
+			continue
+		}
+		if cur == nil {
+			continue
+		}
+		switch {
+		case strings.HasPrefix(line, "--- a/"):
+			cur.OldPath = strings.TrimPrefix(line, "--- a/")
+		case strings.HasPrefix(line, "+++ b/"):
+			cur.Path = strings.TrimPrefix(line, "+++ b/")
+		case strings.HasPrefix(line, "+") && !strings.HasPrefix(line, "+++"):
+			cur.Additions++
+			patchLines = append(patchLines, line)
+		case strings.HasPrefix(line, "-") && !strings.HasPrefix(line, "---"):
+			cur.Deletions++
+			patchLines = append(patchLines, line)
+		default:
+			patchLines = append(patchLines, line)
+		}
+	}
+	commit()
+	return files
+}