Default editor for git set to nano--how?
You are missing GIT_EDITOR and core.editor.
ENVIRONMENT AND CONFIGURATION VARIABLES
The editor used to edit the commit log message will be chosen from the
GIT_EDITORenvironment variable, thecore.editorconfiguration variable, theVISUALenvironment variable, or theEDITORenvironment variable (in that order).
You set core.editor with ...
git config --global core.editor "vim"
My wild guess you have set nano as a default system editor, but you can try to update it with
sudo update-alternatives --config editor