🐛 fix broken git repo support where gitfs2 is not installed

[chfw]

Before raising the PR, here is a check list:

• have you updated the change log?
• can someone else understand your changes without your explanation?
• are you proud of your code changes?

fix #328

[codecov-io]

Codecov Report

Merging #329 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev     #329   +/-   ##
=======================================
  Coverage   95.36%   95.36%           
=======================================
  Files          51       51           
  Lines        2547     2547           
=======================================
  Hits         2429     2429           
  Misses        118      118

Impacted Files	Coverage Δ
moban/_version.py	100% <100%> (ø)	⬆️

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