worktree add: preserve version metadata for unmodified files on dvc add

[pmrowla]

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Fixes #8293

[pmrowla]

Continually adding fields to these tuples was not ideal as far as maintaining it goes, is clearer/easier to update in the future by just keeping these assignments in Output

[codecov]

Codecov Report

Base: 94.28% // Head: 94.28% // No change to project coverage 👍

Coverage data is based on head (77ae4aa) compared to base (77ae4aa).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8595   +/-   ##
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  Coverage   94.28%   94.28%           
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  Files         431      431           
  Lines       32931    32931           
  Branches     4607     4607           
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  Hits        31048    31048           
  Misses       1461     1461           
  Partials      422      422           

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[pmrowla]

This extra reload on save might be expensive, so it's tied to the merge_versioned flag for now.

The problem is that we can't do this in the same place we are doing restore_fields. restore_fields is done on repo.stage.create and is intended to set fields that may be replaced/updated afterwards (i.e. we load original annotations from a dvcfile in restore_fields and then use out.annot.update() later to replace the loaded values with any new/updated ones from the command line.

But for the version metadata merge, we cannot do it until after the new outs have been save()'d, since we need the new out md5's to be computed before we can compare them to the old ones.