anago fails to rebase during release phase ('gitlib::push_master') on master branch

[justaugustus]

What happened:
While testing the updated kube-cross locations in #1141, I was interested in two scenarios:

• Correctly using the kube-cross:v1.12.17-1 image
• Correctly using the kube-cross:v1.13.8-1 image

For the first, I ran a stage and release against the release-1.15 branch:

$ TOOL_ORG=justaugustus \
TOOL_BRANCH=kcb-image \
./gcbmgr stage release-1.15 --buildversion=$(curl -Ls https://dl.k8s.io/ci/latest-1.15.txt)

$ TOOL_ORG=justaugustus \
TOOL_BRANCH=kcb-image \
./gcbmgr release release-1.15 --buildversion=v1.15.11-beta.0.18+89a7468341d588

Both jobs were successful.

For the kube-cross:v1.13.8-1 image, I used the master branch.

Stage:

$ TOOL_ORG=justaugustus \
TOOL_BRANCH=kcb-image \
./gcbmgr stage master --buildversion=$(curl -Ls https://dl.k8s.io/ci/latest.txt)

Release:

$ TOOL_ORG=justaugustus \
TOOL_BRANCH=kcb-image \
./gcbmgr release master --buildversion=v1.19.0-alpha.0.393+debb1edee1d4d2

The staging job ran successfully, but the release one failed with:

Step #2: ================================================================================
Step #2: PUSH GIT OBJECTS  (11/14)
Step #2: ================================================================================
Step #2: 
Step #2: Checkout master branch to push objects: OK
Step #2: Pushing --dry-run tags
Step #2: Pushing v1.19.0-alpha.1 tag: OK
Step #2: Checkout master branch to push objects: OK
Step #2: Rebase master branch: FAILED
Step #2: [2020-Mar-01 01:34:32 UTC] push_git_objects in 4s
Step #2: FAILED in push_git_objects.
Step #2: 
Step #2: RELEASE INCOMPLETE! Exiting...

Fairly certain this has to do with the changelog.
I'm working on fixing this in #1143.

/assign
cc: @cpanato @saschagrunert
/priority critical-urgent

[justaugustus]

#1143 is ready for review/approval.