Call pre_save_hook only on first chunk of large files

[davidbrochart]

See #696 (comment).
cc @kevin-bates

[codecov-commenter]

Codecov Report

Merging #716 (e6ca20f) into main (426a4c6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #716   +/-   ##
=======================================
  Coverage   70.84%   70.84%           
=======================================
  Files          62       62           
  Lines        7490     7491    +1     
  Branches     1187     1188    +1     
=======================================
+ Hits         5306     5307    +1     
  Misses       1829     1829           
  Partials      355      355           

Impacted Files	Coverage Δ
jupyter_server/services/contents/filemanager.py	70.62% <100.00%> (ø)
...pyter_server/services/contents/largefilemanager.py	83.15% <100.00%> (+0.36%)	⬆️
jupyter_server/serverapp.py	65.38% <0.00%> (-0.13%)	⬇️
jupyter_server/extension/application.py	74.66% <0.00%> (+0.34%)	⬆️

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[kevin-bates]

Thanks @davidbrochart.