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
running: init test1
result: None
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: None
running: commit -m commit2 -d 2000-01-02 foo
result: None
running: log -r 0
changeset: 0:0e4634943879
user: test
date: Sat Jan 01 00:00:00 2000 +0000
summary: commit1
result: None
running: log -r tip
changeset: 1:45589e459b2e
tag: tip
user: test
date: Sun Jan 02 00:00:00 2000 +0000
summary: commit2
result: None