diff --git a/git-difftool.perl b/git-difftool.perl
index 18ca61e8d0..598fcc23b9 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -47,13 +47,9 @@ sub find_worktree
 
 sub print_tool_help
 {
-	my $cmd = 'TOOL_MODE=diff';
-	$cmd .= ' && . "$(git --exec-path)/git-mergetool--lib"';
-	$cmd .= ' && show_tool_help';
-
 	# See the comment at the bottom of file_diff() for the reason behind
 	# using system() followed by exit() instead of exec().
-	my $rc = system('sh', '-c', $cmd);
+	my $rc = system(qw(git mergetool --tool-help=diff));
 	exit($rc | ($rc >> 8));
 }
 
diff --git a/git-mergetool.sh b/git-mergetool.sh
index 96a61ba6f4..ec644d542d 100755
--- a/git-mergetool.sh
+++ b/git-mergetool.sh
@@ -10,11 +10,11 @@
 
 USAGE='[--tool=tool] [--tool-help] [-y|--no-prompt|--prompt] [file to merge] ...'
 SUBDIRECTORY_OK=Yes
+NONGIT_OK=Yes
 OPTIONS_SPEC=
 TOOL_MODE=merge
 . git-sh-setup
 . git-mergetool--lib
-require_work_tree
 
 # Returns true if the mode reflects a symlink
 is_symlink () {
@@ -327,6 +327,10 @@ guessed_merge_tool=false
 while test $# != 0
 do
 	case "$1" in
+	--tool-help=*)
+		TOOL_MODE=${1#--tool-help=}
+		show_tool_help
+		;;
 	--tool-help)
 		show_tool_help
 		;;
@@ -378,6 +382,9 @@ prompt_after_failed_merge () {
 	done
 }
 
+git_dir_init
+require_work_tree
+
 if test -z "$merge_tool"
 then
 	# Check if a merge tool has been configured
diff --git a/git-sh-setup.sh b/git-sh-setup.sh
index 9447980330..d968760139 100644
--- a/git-sh-setup.sh
+++ b/git-sh-setup.sh
@@ -330,8 +330,7 @@ esac
 
 # Make sure we are in a valid repository of a vintage we understand,
 # if we require to be in a git repository.
-if test -z "$NONGIT_OK"
-then
+git_dir_init () {
 	GIT_DIR=$(git rev-parse --git-dir) || exit
 	if [ -z "$SUBDIRECTORY_OK" ]
 	then
@@ -346,6 +345,11 @@ then
 		exit 1
 	}
 	: ${GIT_OBJECT_DIRECTORY="$GIT_DIR/objects"}
+}
+
+if test -z "$NONGIT_OK"
+then
+	git_dir_init
 fi
 
 peel_committish () {
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 2834ded84d..3502ec9fe5 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -14,7 +14,7 @@ Testing basic merge tool invocation'
 # running mergetool
 
 test_expect_success 'setup' '
-	git config rerere.enabled true &&
+	test_config rerere.enabled true &&
 	echo master >file1 &&
 	echo master spaced >"spaced name" &&
 	echo master file11 >file11 &&
@@ -129,7 +129,7 @@ test_expect_success 'mergetool crlf' '
 	git submodule update -N &&
 	test "$(cat submod/bar)" = "master submodule" &&
 	git commit -m "branch1 resolved with mergetool - autocrlf" &&
-	git config core.autocrlf false &&
+	test_config core.autocrlf false &&
 	git reset --hard
 '
 
@@ -176,7 +176,7 @@ test_expect_success 'mergetool skips autoresolved' '
 test_expect_success 'mergetool merges all from subdir' '
 	(
 		cd subdir &&
-		git config rerere.enabled false &&
+		test_config rerere.enabled false &&
 		test_must_fail git merge master &&
 		( yes "r" | git mergetool ../submod ) &&
 		( yes "d" "d" | git mergetool --no-prompt ) &&
@@ -190,7 +190,7 @@ test_expect_success 'mergetool merges all from subdir' '
 '
 
 test_expect_success 'mergetool skips resolved paths when rerere is active' '
-	git config rerere.enabled true &&
+	test_config rerere.enabled true &&
 	rm -rf .git/rr-cache &&
 	git checkout -b test5 branch1 &&
 	git submodule update -N &&
@@ -204,7 +204,7 @@ test_expect_success 'mergetool skips resolved paths when rerere is active' '
 '
 
 test_expect_success 'conflicted stash sets up rerere'  '
-	git config rerere.enabled true &&
+	test_config rerere.enabled true &&
 	git checkout stash1 &&
 	echo "Conflicting stash content" >file11 &&
 	git stash &&
@@ -232,7 +232,7 @@ test_expect_success 'conflicted stash sets up rerere'  '
 
 test_expect_success 'mergetool takes partial path' '
 	git reset --hard &&
-	git config rerere.enabled false &&
+	test_config rerere.enabled false &&
 	git checkout -b test12 branch1 &&
 	git submodule update -N &&
 	test_must_fail git merge master &&