Before, --tool=:builtin argument was ignored and the tool was loaded from
"ui.diff.tool" option. Since there is no single builtin diff format, :builtin
doesn't make sense here. Maybe we can translate ":<format>" to the internal
diff format instead, but that will also mean "ui.diff.tool" and ".format" can
be merged.
This partially reverts 409356fa5b "merge_tools: enable `:builtin` as default
diff/merge editor."