Update nested config lookup to ignore certain directories

[jacebrowning]

This will speed up the initial install and update commands.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.45%. Comparing base (7c9f462) to head (11c1f0a).
Report is 20 commits behind head on main.

Files	Patch %	Lines
gitman/models/config.py	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
+ Coverage   91.35%   91.45%   +0.09%     
==========================================
  Files          15       15              
  Lines        1192     1193       +1     
  Branches      242      243       +1     
==========================================
+ Hits         1089     1091       +2     
  Misses         70       70              
+ Partials       33       32       -1     

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