When a git mirror is configured, it fetches only refs/heads/*,
and possibly branch-heads and tags, but not refs/changes.

When gclient attempts to sync refs/changes/* from a mirror,
it fails, since the mirror has no such objects.

See for example [1], where the DEPS entry for builtools was
modified to sync to a CL, but all the bots failed to sync to it.

This change modifies gclient to always fetch refs/changes/*
directly from the repo, even when a mirror is configured.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1119098/6

Bug: 858894

Change-Id: I71bb313e4325a81b2985db1d00c70a8844dc7c22
Reviewed-on: https://chromium-review.googlesource.com/1119525
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>