Fix download charts after a DST transition

When falling back, a bug in date-fns[^0] results in the 23 hour day
being skipped when building the `dates` array in `toChartData` if the
current time is between 00:00 and 01:00 UTC. This results in that day's
data essentially going missing, resulting in the subtly broken charts
noted in rust-lang#5477.

This commit adds a workaround adapted from a comment on
date-fns/date-fns#571[^1] by @bertho-zero, which correctly adjusts the
new date based on the time zone offsets, and means that `dates` is built
as expected.

Fixes rust-lang#5477.

[^0]: date-fns/date-fns#571
[^1]: date-fns/date-fns#571 (comment)

app/components/download-graph.js

@@ -3,7 +3,7 @@ import { inject as service } from '@ember/service';
 import { waitForPromise } from '@ember/test-waiters';
 import Component from '@glimmer/component';
 
-import subDays from 'date-fns/subDays';
+import { addMinutes, subDays as brokenSubDays, subMinutes } from 'date-fns';
 import window from 'ember-window-mock';
 import semverSort from 'semver/functions/sort';
 
@@ -190,3 +190,18 @@ export function toChartData(data) {
 function midnightForDate(date) {
   return Date.parse(date.toISOString().slice(0, 10));
 }
+
+// This works around a bug in date-fn's subDays() function when crossing a DST
+// transition: https://github.com/date-fns/date-fns/issues/571
+//
+// The specific implementation is based on @bertho-zero's here:
+// https://github.com/date-fns/date-fns/issues/571#issuecomment-602496322
+function subDays(date, amount) {
+  const originalTZO = date.getTimezoneOffset();
+  const endDate = brokenSubDays(date, amount);
+  const endTZO = endDate.getTimezoneOffset();
+
+  const dstDiff = originalTZO - endTZO;
+
+  return dstDiff >= 0 ? addMinutes(endDate, dstDiff) : subMinutes(endDate, Math.abs(dstDiff));
+}