Add a regression test for an idempotency bug. (#1752)

Idempotency bugs where the formatter produces another change if you
re-run it on its own output are like Whac-a-mole. We don't have a
systematic way to prevent them, so I just fix them as they come up.

We do *test* for them systematically now. Every single test is formatted
twice, again on the output of the first format to ensure that what we
test is at least idempotent.

This one appears to no longer be an issue because the test is already
passing. Probably got fixed in some of the changes around comment
handling.

This just adds a regression test.

Close #1625.

Author: munificent

test/tall/regression/1600/1625.unit

@@ -0,0 +1,17 @@
+>>>
+bool foo() {
+  if (
+  // Explicitly stated to not be a bot.
+  Platform.environment['BOT'] == 'false'
+  ) {
+    return false;
+  }
+}
+<<<
+bool foo() {
+  if (
+  // Explicitly stated to not be a bot.
+  Platform.environment['BOT'] == 'false') {
+    return false;
+  }
+}