repository and revision have less to do with each other than it seems:
revision is for checkout and the part of code that reads revision does not
read repository.
repository property is used to determine what to apply the patchset to.

When initializing their values from buildbucket.build.input, do it separately.
Also, initialize repository from Gerrit changes, not Gitiles commit.

See also
https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1199854
https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/1199856
Bug: 877161, 694348
Change-Id: I4907a31ce055a3526b15f1a3c8e8f15c2c831437
Reviewed-on: https://chromium-review.googlesource.com/1200065Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>