Escape original merge branch to compile regex

[rrahn]

This PR makes the regex pattern more stable, e.g. if local version specifiers indicated by '+' sign are added to the version. Such a plus sign, however, would cause the regex to fail as it interprets the plus as a regex quantifier.

PR checklist

• This comment contains a description of changes (with reason)
• CHANGELOG.md is updated
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.40%. Comparing base (6d9d4f7) to head (72c6f46).
⚠️ Report is 6 commits behind head on dev.

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