This is not a command the end user would want to run. Ever. This documentation is meant for people who are studying the Porcelain-ish scripts and/or are writing new ones.
The git-mergetool--lib scriptlet is designed to be sourced (using .) by other shell scripts to set up functions for working with Git merge tools.
Before sourcing git-mergetool--lib, your script must set TOOL_MODE to define the operation mode for the functions listed below. diff and merge are valid values.