Fix rosdep file targeting with head_ref argument

[cottsay]

When we're using head_ref, we should determine what rosdep files exist in the tree behind that ref, and not which files are present in the working directory.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.

Project coverage is 38.06%. Comparing base (5e0a145) to head (756231d).

Files with missing lines	Patch %	Lines
rosdistro_reviewer/element_analyzer/rosdep.py	0.00%	16 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
- Coverage   39.10%   38.06%   -1.05%     
==========================================
  Files          12       12              
  Lines         583      599      +16     
  Branches      154      158       +4     
==========================================
  Hits          228      228              
- Misses        350      365      +15     
- Partials        5        6       +1     

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