Issue 882: add standard hook to reject text files with CRLF.
While the win32text extension does LF <-> CRLF conversion, and will issue a
warning in case a file already in the repository uses CRLF, it provides no
mechanism for verifying that incoming changes use LF. In a large development
team with some Windows users, it is virtually guaranteed that someone will
forget to set up the encode filter correctly and accidentally check in a file
using CRLF, which can cause warnings for other Windows users when they next
fetch changes. Since this is a general problem it is desirable to have a
pre-commit (or -push) hook available to reject such accidents earlier rather
than trying to fix them up after the fact.