Ensure size_hint never exceeds graph_limit

If we have thousands of syntax nodes on both sides, we can end
up attempting to preallocate a very large hashmap.

In #542, a user hit an issue with two JSON files where the LHS had
33,000 syntax nodes and the RHS had 34,000 nodes, so we'd attempt to
preallocate a hashmap of capacity 1,122,000,000. This required
allocating 70,866,960,400 bytes (roughly 66 GiB).

Impose a sensible limit on the hashmap.

Fixes #542

CHANGELOG.md

@@ -7,6 +7,11 @@ prominent.
 
 Improved syntax hightling for Java built-in types.
 
+### Diffing
+
+Fixed an issue with runaway memory usage when the two files input
+files had a large number of differences.
+
 ## 0.49 (release 26th July 2023)
 
 ### Parsing

src/diff/dijkstra.rs

@@ -205,7 +205,12 @@ pub fn mark_syntax<'a>(
     // graph whose size is roughly quadratic. Use this as a size hint,
     // so we don't spend too much time re-hashing and expanding the
     // predecessors hashmap.
-    let size_hint = lhs_node_count * rhs_node_count;
+    //
+    // Cap this number to the graph limit, so we don't try to allocate
+    // an absurdly large (i.e. greater than physical memory) hashmap
+    // when there is a large number of nodes. We'll never visit more
+    // than graph_limit nodes.
+    let size_hint = std::cmp::min(lhs_node_count * rhs_node_count, graph_limit);
 
     let start = Vertex::new(lhs_syntax, rhs_syntax);
     let vertex_arena = Bump::new();