T7319: check for unsaved commits before proceeding with 'add system image'

[jestabro]

Change summary

DRAFT state until merge of required #4864 and a rebase. After rebase, this PR will consist only of the final commit below.

On add system image, exit with warning message if there are unsaved commits.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

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Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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[github-actions]

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