Don't flatten error nodes

As of b9d44ae (released in 0.38), difftastic prefers a textual
diff if any parse errors are present. As a result, users have to
opt-in to allowing parse errors. We can now preserve structure to
produce diffs that are more granular, at the slight risk of changes
being missed if parsers discard tokens.

Fixes #703

CHANGELOG.md

@@ -1,5 +1,17 @@
 ## 0.58 (unreleased)
 
+### Parsing
+
+Difftastic now preserves tree-sitter parse tree structure on parse
+error nodes. This reverts the flattening behaviour introduced in
+0.38. Preserving structure tends to produce better diffs, although it
+increases the risk that difftastic will show fewer changes in the
+presence of parse errors.
+
+Since difftastic is now conservative with parse errors
+(DFT_PARSE_ERROR_LIMIT is 0 by default), this seems like a better
+tradeoff.
+
 ### Diffing
 
 `--strip-cr` now defaults to `on`, so comparing a file with CRLF

sample_files/compare.expected

@@ -134,7 +134,7 @@ sample_files/lua_before.lua sample_files/lua_after.lua
 9f5c85cd6806c724c84afa805da76bb7  -
 
 sample_files/makefile_before.mk sample_files/makefile_after.mk
-7eea0d919b71161c72a4ef496fff749d  -
+20cf6b508d1707e41e5450572ee51838  -
 
 sample_files/many_newlines_before.txt sample_files/many_newlines_after.txt
 afc8628f89076423d1453d8e3c84ac92  -
@@ -254,7 +254,7 @@ sample_files/tab_before.txt sample_files/tab_after.txt
 c7fcee21dd5ebd251e2cf4cd7d4446d2  -
 
 sample_files/tailwind_before.css sample_files/tailwind_after.css
-db01dde1e8455a144e5bf0511273f674  -
+be0cba204957b62a30e99fd03f945de5  -
 
 sample_files/text_before.txt sample_files/text_after.txt
 5fa6a472ccdeda0142eda3edcaeaa189  -

src/parse/tree_sitter_parser.rs

@@ -1608,10 +1608,9 @@ fn syntax_from_cursor<'a>(
 
     if node.is_error() {
         *error_count += 1;
+    }
 
-        // Treat error nodes as atoms, even if they have children.
-        atom_from_cursor(arena, src, nl_pos, cursor, highlights, ignore_comments)
-    } else if config.atom_nodes.contains(node.kind()) {
+    if config.atom_nodes.contains(node.kind()) {
         // Treat nodes like string literals as atoms, regardless
         // of whether they have children.
         atom_from_cursor(arena, src, nl_pos, cursor, highlights, ignore_comments)