bazelci.py: fix get_modified_files (#1886)

We need to diff against the merge base commit from the base branch to
get the full list of changed files in a PR.

For example, in
https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/20282#018dccf4-3a59-40c4-b8ac-1058aaa072d9,
most of tests were accidentally filtered out because the latest commit
only contains one BUILD file change.
1 file changed