fix(file): handle file diff overwrite properly

[neilime]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 89.79592% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.72%. Comparing base (c67c248) to head (0001974).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/services/file/FileDiffService.ts	81.48%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #757      +/-   ##
==========================================
+ Coverage   65.55%   65.72%   +0.17%     
==========================================
  Files          77       77              
  Lines        2000     2019      +19     
  Branches      348      352       +4     
==========================================
+ Hits         1311     1327      +16     
- Misses        689      692       +3     

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