Fallback to start commit when merge base is missing

Vasilii Iakliushinrequested to merge
407552_fix_compare_logic_for_missing_merge_base into master

What does this MR do and why?

Contributes to #407552 (closed)

Problem

When two branches don't have a common merge base Compare API returns a 500 error.

Solution

Fallback to start commit when the common merge base is missing.

How to set up and validate locally

  1. Clone https://gitlab.com/vyaklushin/compare-diverged-test
  2. Visit compare page: https://gitlab.com/vyaklushin/compare-diverged-test/-/compare/diverge...master?from_project_id=45262106&straight=false
  3. Visit compare API (on GitLab.com raises 500 error): https://gitlab.com/api/v4/projects/45262106/repository/compare?from=diverge&to=master
  4. After the fix, the API response should be rendered and match compare page results.

MR acceptance checklist

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

Edited by Vasilii Iakliushin

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