feat: terramate generate should not overwrite files that are not terramate managed

[katcipis]

For now using a perfect match with the generated header as a way to detect if the file is a terramate generated file or not. If it is not a terramate generated file now we fail generation for the specific stack + file remains unchanged.

[codecov-commenter]

Codecov Report

Merging #134 (efe622f) into main (0012dde) will increase coverage by 0.06%.
The diff coverage is 85.41%.

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   74.29%   74.35%   +0.06%     
==========================================
  Files          29       29              
  Lines        3077     3108      +31     
==========================================
+ Hits         2286     2311      +25     
- Misses        593      598       +5     
- Partials      198      199       +1     

Flag	Coverage Δ
tests	74.35% <85.41%> (+0.06%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
test/sandbox/sandbox.go	83.56% <83.33%> (+0.70%)	⬆️
generate/generate.go	87.50% <86.66%> (-0.82%)	⬇️
git/git.go	65.89% <0.00%> (-0.52%)	⬇️

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[mariux]

lgtm