Found the setup by comparing the configuration of the Git extension with one of my colleagues:
<key> <string>fullhistoryinfilehistory</string> </key> <value> <string>true</string> </value>
I set the value to false, now it works as intended. Cannot find a GUI equivalent option in Git Extension, though ...
Edit: found! Not in the settings, but in the context menu in the file history → Full history: 
rotsch
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