difftool: honor --trust-exit-code for builtin tools

run_merge_tool() was not setting $status, which prevented the
exit code for builtin tools from being forwarded to the caller.

Capture the exit status and add a test to guarantee the behavior.

Reported-by: Adria Farres <14farresa@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-mergetool--lib.sh

@@ -221,6 +221,7 @@ run_merge_tool () {
 	else
 		run_diff_cmd "$1"
 	fi
+	status=$?
 	return $status
 }
 

t/t7800-difftool.sh

@@ -86,6 +86,11 @@ test_expect_success PERL 'difftool forwards exit code with --trust-exit-code' '
 	test_must_fail git difftool -y --trust-exit-code -t error branch
 '
 
+test_expect_success PERL 'difftool forwards exit code with --trust-exit-code for built-ins' '
+	test_config difftool.vimdiff.path false &&
+	test_must_fail git difftool -y --trust-exit-code -t vimdiff branch
+'
+
 test_expect_success PERL 'difftool honors difftool.trustExitCode = true' '
 	test_config difftool.error.cmd false &&
 	test_config difftool.trustExitCode true &&