Merge branch 'jk/difftool-command-not-found' into maint

"git difftool" by default ignores the error exit from the backend
commands it spawns, because often they signal that they found
differences by exiting with a non-zero status code just like "diff"
does; the exit status codes 126 and above however are special in
that they are used to signal that the command is not executable,
does not exist, or killed by a signal.  "git difftool" has been
taught to notice these exit status codes.

* jk/difftool-command-not-found:
  difftool: always honor fatal error exit codes

git-difftool--helper.sh

@@ -86,6 +86,13 @@ else
 	do
 		launch_merge_tool "$1" "$2" "$5"
 		status=$?
+		if test $status -ge 126
+		then
+			# Command not found (127), not executable (126) or
+			# exited via a signal (>= 128).
+			exit $status
+		fi
+
 		if test "$status" != 0 &&
 			test "$GIT_DIFFTOOL_TRUST_EXIT_CODE" = true
 		then

t/t7800-difftool.sh

@@ -124,6 +124,12 @@ test_expect_success PERL 'difftool stops on error with --trust-exit-code' '
 	test_cmp expect actual
 '
 
+test_expect_success PERL 'difftool honors exit status if command not found' '
+	test_config difftool.nonexistent.cmd i-dont-exist &&
+	test_config difftool.trustExitCode false &&
+	test_must_fail git difftool -y -t nonexistent branch
+'
+
 test_expect_success PERL 'difftool honors --gui' '
 	difftool_test_setup &&
 	test_config merge.tool bogus-tool &&