Fix temp git clones clutter issue.

[BoubQC]

Use filesystem.safe_delete_folder instead of shutil.rmtree since the later doesn't delete read-only files.

[codecov]

Codecov Report

Merging #845 (e74fe51) into master (4f3880b) will decrease coverage by 6.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #845      +/-   ##
==========================================
- Coverage   79.38%   72.82%   -6.56%     
==========================================
  Files         177      177              
  Lines       19302    19302              
==========================================
- Hits        15322    14056    -1266     
- Misses       3980     5246    +1266     

Impacted Files	Coverage Δ
python/tank/descriptor/io_descriptor/git.py	86.17% <100.00%> (ø)
python/tank/commands/app_info.py	17.24% <0.00%> (-79.32%)	⬇️
python/tank/bootstrap/import_handler.py	18.18% <0.00%> (-73.74%)	⬇️
python/tank/commands/folders.py	20.00% <0.00%> (-69.10%)	⬇️
python/tank/commands/setup_project.py	17.32% <0.00%> (-66.29%)	⬇️
python/tank/commands/cache_yaml.py	31.57% <0.00%> (-52.64%)	⬇️
python/tank/commands/util.py	37.50% <0.00%> (-37.50%)	⬇️
python/tank/commands/core_localize.py	39.40% <0.00%> (-36.44%)	⬇️
python/tank/commands/unregister_folders.py	12.58% <0.00%> (-36.43%)	⬇️
python/tank/commands/tank_command.py	47.34% <0.00%> (-32.98%)	⬇️
... and 43 more

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[staceyoue]

LGTM!