[gitlib] use shared test helpers

[hoegaarden]

/assign @justaugustus
/kind cleanup

[k8s-ci-robot]

@hoegaarden: Adding label: do-not-merge/blocked-paths because PR changes a protected file.

Reasons for blocking this PR:

[Changes to certain release tools can affect our ability to test, build, and release Kubernetes. This PR must be explicitly approved by SIG Release repo admins.]

• lib/gitlib_test.sh

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[justaugustus]

/lgtm
/approve
/priority important-soon
/milestone v1.16

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hoegaarden, justaugustus

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