Introduce git__date_rfc2822_fmt. Allows for RFC2822 date headers
Added RFC2822 date format test cases
Fix const-correctness of git_patch_get_delta, git_patch_num_hunks, git_patch_num_lines_in_hunk
Add test files
Introduce git_diff_get_stats and git_diff_stats_to_buf
Introduce git_diff_format_email and git_diff_commit_as_email