This patch fixes 'hg diff' output for me but not 'hg export', where I still get
the CR-CR-LF line endings.

However, it seems this bug should be fixed at the same time as a related one in
'hg import'.  Without hacking mercurial.patch.patch to include --binary I can
import a "correct" patch (i.e., one with CR-LF line endings) but the output uses
LF endings.  With --binary, I can't import a "correct" patch any more, but an
CR-CR-LF one will import as just CR-LF and work fine.