Add Patchset.RefsChanges(). Returns the 'refs/changes/X/Y/Z' string
for the given patchset.
Add []FileComment parameter to Gerrit.Comment(), allowing the posting
of comments on particular lines of the change.
Bug: dawn:1342
Change-Id: Ia4113a158b285c606b2c6c520cef6c9e8030fae7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/88314
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
526087bde7