Fix base path to avoid duplicate parts in path when deleting reference

[pedrosmr]

I opened up an issue for a bug that's raising a KeyError when using allOf with relative reference to schema in same directory.

This PR applies the changes mentioned there to hopefully resolve the issue.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e9b6edf) 100.00% compared to head (f5d13ed) 100.00%.
Report is 720 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##            master     #1550     +/-   ##
===========================================
  Coverage   100.00%   100.00%             
===========================================
  Files           11        33     +22     
  Lines         1020      3622   +2602     
  Branches       201       842    +641     
===========================================
+ Hits          1020      3622   +2602     

Flag	Coverage Δ
unittests	99.64% <99.59%> (?)

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Files Changed	Coverage Δ
datamodel_code_generator/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/__main__.py	100.00% <100.00%> (ø)
datamodel_code_generator/arguments.py	100.00% <100.00%> (ø)
datamodel_code_generator/format.py	100.00% <100.00%> (ø)
datamodel_code_generator/http.py	100.00% <100.00%> (ø)
datamodel_code_generator/imports.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/__init__.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/base.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/dataclass.py	100.00% <100.00%> (ø)
datamodel_code_generator/model/enum.py	100.00% <100.00%> (ø)
... and 23 more

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