Fix abs_pos computing in String_zipper.drop_until

[emilienlemaire]

This PR should fix a problem that occurs when editing a text document with apply_text_document_edits, where some text could be duplicated, as shown in #1207.

[coveralls]

Pull Request Test Coverage Report for Build 4250

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Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 21.189%

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Totals
Change from base Build 4249:	0.0%
Covered Lines:	5147
Relevant Lines:	24291

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💛 - Coveralls

[rgrinberg]

Thanks for the fix. In fact, this PR is two independent fixes and I would like tests for each individual fix separately. The abs_pos fix is in fact not covered by the tests, as you can see here: #1280

EDIT: managed to add a separate test for abs_pos management

[rgrinberg]

Thanks!