branch_util: remove main/master hardcode for manifest-versions

We can use HEAD to get the latest correct value.

BUG=b:173131586
TEST=CQ passes

Change-Id: I38cb0297a8514aa5cd51c50d0b65ce6cd54faf0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2601658
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Julio Hurtado <juahurta@google.com>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed