Introduce git_diff_get_stats, git_diff_stats_files_changed, git_diff_stats_insertions, git_diff_stats_deletions and git_diff_stats_to_buf
Introduce git_diff_format_email and git_diff_commit_as_email
Sanitize git_diff_format_email_options' summary parameter

It will form part of the subject line and should thus be one line.