improve merge worflow

[davidB]

[ ] display a message if git mergetool is not defined (or doesn't propose to merge file) [ ] on merge, generate a MERGED file with conflict by using the diff function and follow the rule from https://git-scm.com/docs/git-merge#_how_conflicts_are_presented & https://stackoverflow.com/questions/44549733/how-to-use-visual-studio-code-as-default-editor-for-git-mergetool [ ] if vscode is installed (code is in the path) then use it for merge code --wait $MERGED (see https://medium.com/faun/using-vscode-as-git-mergetool-and-difftool-2e241123abe7)