Skip long arguments in syntax highlighting path detection

When fish performs syntax highlighting, it attempts to determine which
arguments are valid paths and underline them. Skip paths whose length
exceeds PATH_MAX. This is an optimization: such strings are almost
certainly not valid paths and checking them may be expensive.

Relevant is #7837

src/highlight.cpp

@@ -918,6 +918,11 @@ void highlighter_t::color_as_argument(const ast::node_t &node) {
 static bool range_is_potential_path(const wcstring &src, const source_range_t &range,
                                     const operation_context_t &ctx,
                                     const wcstring &working_directory) {
+    // Skip strings exceeding PATH_MAX. See #7837.
+    // Note some paths may exceed PATH_MAX, but this is just for highlighting.
+    if (range.length > PATH_MAX) {
+        return false;
+    }
     // Get the node source, unescape it, and then pass it to is_potential_path along with the
     // working directory (as a one element list).
     bool result = false;