branch_util: Fix up logging

Stdout was being captured in a weird way so informational messages
were being logged to stderr. This change better organizes log
messages.

BUG=b:177903295
TEST=none

Change-Id: I3117e878972683fdd543bc242d6d0cc3deccb743
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2698795
Reviewed-by: Julio Hurtado <juahurta@google.com>
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>