Use the same algorithm to recognize merged branches in `delete-unmanaged` as for `status` and `traverse`

[PawelLipski]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.42%. Comparing base (73aba92) to head (1b56201). Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1313      +/-   ##
===========================================
+ Coverage    98.40%   98.42%   +0.02%     
===========================================
  Files           14       14              
  Lines         3883     3878       -5     
  Branches       849      848       -1     
===========================================
- Hits          3821     3817       -4     
+ Misses          40       39       -1     
  Partials        22       22              

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