rust-matchers: better support file_set in IntersectionMatcher

Previously, if both of the input matchers had non-exact file sets,
IntersectionMatcher would simply have an empty fileset.

Now, we duplicate Python behavior: we *union* the filesets of the
matchers in that case.  This makes some sense, because without exact
input file sets, we can't do an exact intersection and must
over-approximate.

Concretely, this is necessary because the file_set is the list of
files that were explicitly listed by the user, and we want to provide
explicit errors for all such files, from both matchers.

rust/hg-core/src/matchers.rs

@@ -479,7 +479,13 b' impl IntersectionMatcher {'
                 m1_files.iter().cloned().filter(|f| m2.matches(f)).collect()
             })
         } else {
-            None
+            // without exact input file sets, we can't do an exact
+            // intersection, so we must over-approximate by
+            // unioning instead
+            m1.file_set().map(|m1_files| match m2.file_set() {
+                Some(m2_files) => m1_files.union(m2_files).cloned().collect(),
+                None => m1_files.iter().cloned().collect(),
+            })
         };
         Self { m1, m2, files }
     }