test-check-code: do not use xargs

We have too many files, and passing them via arguments could cause strange
errors on some platforms [1]. Since check-code.py can now take "-" and read
file names from stdin, use it instead of xargs to avoid the argv size limit.

[1]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-April/096346.html

tests/test-check-code.t

@@ -8,7 +8,7 b' New errors are not allowed. Warnings are'
 (The writing "no-che?k-code" is for not skipping this file when checking.)
 
   $ hg locate -X contrib/python-zstandard -X hgext/fsmonitor/pywatchman |
-  > sed 's-\\-/-g' | xargs "$check_code" --warnings --per-file=0 || false
+  > sed 's-\\-/-g' | "$check_code" --warnings --per-file=0 - || false
   contrib/perf.py:869:
    >             r.revision(r.node(x))
    don't convert rev to node before passing to revision(nodeorrev)