Refactor root sha access code

What does this MR do and why?

Contributes to #391900 (closed)

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1619/?project=3

Problem

We raise, handle an exception and report an error to Sentry without a reason. It can be done outside of exception handling flow.

Solution

Verify if the root commit exists before requesting its sha.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.