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This PR fixes the incremental->differential upgrade behaviour: Currently if an incremental job is upgraded to a differential time, its since time is not updated to reflect the fact that it should not build upon the last incremental
but instead build upon the last full/differential (whichever is newer).

Please check

• Short description and the purpose of this PR is present above this paragraph
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If you have any questions or problems, please give a comment in the PR.

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Checklist for the reviewer of the PR (will be processed by the Bareos team)

Make sure you check/merge the PR using devtools/pr-tool to have some simple automated checks run and a proper changelog record added.

General

• Is the PR title usable as CHANGELOG entry?
• Purpose of the PR is understood
• Commit descriptions are understandable and well formatted
• Required backport PRs have been created
• Correct milestone is set

Source code quality

• Source code changes are understandable
• Variable and function names are meaningful
• Code comments are correct (logically and spelling)
• Required documentation changes are present and part of the PR

Tests

• Decision taken that a test is required (if not, then remove this paragraph)
• The choice of the type of test (unit test or systemtest) is reasonable
• Testname matches exactly what is being tested
• On a fail, output of the test leads quickly to the origin of the fault