test-check-help: fix to work on Windows
The initial problem was `hg files` prints paths with '\', which gets removed
when piped (scanhelptopics.py failed to open 'hgext__init__.py'). Then, xargs
was invoking `hg help` with 'backout\r (esc)', which setting binary mode
prevents.
#require symlink
https://bz.mercurial-scm.org/1438
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch