Add --show-function support

[joshkehn]

Add support for -p/--show-function. This mirror's -p in diff. See #666

I tested this by running the debug build with git diff and checked the output (subset below):

❯ DFT_SHOW_FUNCTION=1 git -c diff.external=target/debug/difft diff 49e5cff6b035431421709dc1f74363d8d14638b9 0e7abbcc3dab432fce1bb0f96ef2a86f03d3979e -- src/main.rs
src/main.rs --- 3/9 --- Rust
577 579 fn check_only_text(
595 597         rhs_src: FileContent::Text(rhs_src.into()),
596 598         lhs_positions: vec![],
597 599         rhs_positions: vec![],
... 600         lhs_fn_spans: vec![],
... 601         rhs_fn_spans: vec![],
598 602         hunks: vec![],
599 603         has_byte_changes,
600 604         has_syntactic_changes: lhs_src != rhs_src,

src/main.rs --- 4/9 --- Rust
604 608 fn diff_file_content(
636 640             rhs_src: FileContent::Text("".into()),
637 641             lhs_positions: vec![],
638 642             rhs_positions: vec![],
... 643             lhs_fn_spans: vec![],
... 644             rhs_fn_spans: vec![],
639 645             hunks: vec![],
640 646             has_byte_changes: None,
641 647             has_syntactic_changes: false,
642 648         };
643 649     }
... 650
... 651     // Populated below (only when --show-function is set and we have a
... 652     // tree-sitter parse) and carried into the DiffResult.
... 653     let mut lhs_fn_spans: FunctionSpans = Vec::new();
... 654     let mut rhs_fn_spans: FunctionSpans = Vec::new();
644 655
645 656     let (file_format, lhs_positions, rhs_positions) = match lang_config {
646 657         None => {

src/main.rs --- 5/9 --- Rust
604 608 fn diff_file_content(
684 695                                     rhs_src: FileContent::Text(rhs_src.to_owned()),
685 696                                     lhs_positions: vec![],
686 697                                     rhs_positions: vec![],
... 698                                     lhs_fn_spans: vec![],
... 699                                     rhs_fn_spans: vec![],
687 700                                     hunks: vec![],
688 701                                     has_byte_changes,
689 702                                     has_syntactic_changes,

src/main.rs --- 6/9 --- Rust
604 608 fn diff_file_content(
744 757                                     rhs_positions.extend(rhs_comments);
745 758                                 }
... 759
... 760                                 if display_options.show_function.is_some() {
... 761                                     lhs_fn_spans = tsp::function_spans(&lhs_tree);
... 762                                     rhs_fn_spans = tsp::function_spans(&rhs_tree);
... 763                                 }
746 764
747 765                                 (
748 766                                     FileFormat::SupportedLanguage(language),