printer: hack in a fix for -l/--files-with-matches when using --pcre2 --multiline with look-around

The underlying issue here is #2528, which was introduced by commit
efd9cfb2fc which fixed another bug.

For the specific case of "did a file match," we can always assume the
match count is at least 1 here. But this doesn't fix the underlying
problem.

Fixes #3139

crates/printer/src/summary.rs

@@ -683,7 +683,11 @@ impl<'p, 's, M: Matcher, W: WriteColor> Sink for SummarySink<'p, 's, M, W> {
                     true
                 },
             )?;
-            count
+            // Because of `find_iter_at_in_context` being a giant
+            // kludge internally, it's possible that it won't find
+            // *any* matches even though we clearly know that there is
+            // at least one. So make sure we record at least one here.
+            count.max(1)
         };
         if is_multi_line {
             self.match_count += sink_match_count;

tests/regression.rs

@@ -1544,3 +1544,44 @@ rgtest!(
         cmd.args(&["--files", "-g", "[abc"]).assert_err();
     }
 );
+
+rgtest!(
+    r3139_multiline_lookahead_files_with_matches,
+    |dir: Dir, _cmd: TestCommand| {
+        // Only PCRE2 supports look-around.
+        if !dir.is_pcre2() {
+            return;
+        }
+        dir.create(
+            "test",
+            "\
+Start \n   \n\n   \
+XXXXXXXXXXXXXXXXXXXXXXXXXX\n   \
+YYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYYY\n   \
+\n      thing2 \n\n",
+        );
+
+        let got = dir
+            .command()
+            .args(&[
+                "--multiline",
+                "--pcre2",
+                r"(?s)Start(?=.*thing2)",
+                "test",
+            ])
+            .stdout();
+        eqnice!("Start \n", got);
+
+        let got = dir
+            .command()
+            .args(&[
+                "--multiline",
+                "--pcre2",
+                "--files-with-matches",
+                r"(?s)Start(?=.*thing2)",
+                "test",
+            ])
+            .stdout();
+        eqnice!("test\n", got);
+    }
+);