-# Allow GIT_DIFF_TOOL and GIT_MERGE_TOOL to provide default values
-test -n "$GIT_MERGE_TOOL" && merge_tool="$GIT_MERGE_TOOL"
-test -n "$GIT_DIFF_TOOL" && merge_tool="$GIT_DIFF_TOOL"
-
-if test -z "$merge_tool"; then
- merge_tool="$(get_merge_tool)" || exit
+if ! use_ext_cmd; then
+ if test -n "$GIT_DIFF_TOOL"; then
+ merge_tool="$GIT_DIFF_TOOL"
+ else
+ merge_tool="$(get_merge_tool)" || exit
+ fi