fs: temporary implement get_file

[efiop]

While waiting for fsspec/filesystem_spec#1191

[codecov-commenter]

Codecov Report

Base: 82.67% // Head: 81.34% // Decreases project coverage by -1.33% ⚠️

Coverage data is based on head (3a6e175) compared to base (4517567).
Patch coverage: 13.63% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #191      +/-   ##
==========================================
- Coverage   82.67%   81.34%   -1.33%     
==========================================
  Files          25       25              
  Lines        3463     3485      +22     
  Branches      597      604       +7     
==========================================
- Hits         2863     2835      -28     
- Misses        509      548      +39     
- Partials       91      102      +11     

Impacted Files	Coverage Δ
src/scmrepo/fs.py	55.55% <13.63%> (-5.84%)	⬇️
src/scmrepo/utils.py	67.74% <0.00%> (-19.36%)	⬇️
tests/conftest.py	86.00% <0.00%> (-9.00%)	⬇️
src/scmrepo/git/backend/pygit2.py	75.86% <0.00%> (-2.18%)	⬇️
src/scmrepo/git/backend/gitpython.py	63.44% <0.00%> (-1.08%)	⬇️
src/scmrepo/git/backend/dulwich/__init__.py	80.84% <0.00%> (-0.47%)	⬇️

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