fix: fix bug in git overwrite promotion step

[krancour]

It was incorrect to ignore .git in the source dir only if it was itself a directory.

Worktrees have a .git file.

This can create problems when copying from a working tree, as .git will be copied, which was not the intention.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.10%. Comparing base (0e173e3) to head (cb8e740).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2588   +/-   ##
=======================================
  Coverage   51.10%   51.10%           
=======================================
  Files         282      282           
  Lines       20945    20945           
=======================================
  Hits        10703    10703           
  Misses       9578     9578           
  Partials      664      664           

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