check-code: fix platform-specific error code variance

test-check-code-hg.t uses xargs to invoke check-code.py on every file in
'hg manifest'. The return code from xargs varies between BSD xargs and
GNU xargs: BSD will return 1 if any invocation exits with an error code;
GNU xargs will return 123 in this case. This normalizes the exit code
back to 1.

tests/test-check-code-hg.t

@@ -8,7 +8,8 b''
 
 New errors are not allowed. Warnings are strongly discouraged.
 
-  $ hg manifest | xargs "$check_code" --warnings --nolineno --per-file=0
+  $ hg manifest | xargs "$check_code" --warnings --nolineno --per-file=0 \
+  > || false
   tests/test-hgweb-raw.t:0:
    >   $ while kill `cat hg.pid` 2>/dev/null; do sleep 0; done
    don't use kill, use killdaemons.py