add config option to exempt specific paths from pre-render branch cleaning

[krancour]

Fixes #210

cc @evgeny-goldin @shelby-moore as we discussed yesterday.

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[codecov]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (a1b3303) 38.64% compared to head (77e0787) 39.53%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #212      +/-   ##
==========================================
+ Coverage   38.64%   39.53%   +0.89%     
==========================================
  Files          19       19              
  Lines        1162     1204      +42     
==========================================
+ Hits          449      476      +27     
- Misses        694      705      +11     
- Partials       19       23       +4     

Files	Coverage Δ
config.go	63.96% <0.00%> (-1.78%)	⬇️
branches.go	33.54% <66.66%> (+11.40%)	⬆️

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[alexmt]

LGTM!