[BUG?] applypatch-msg hook no-longer thinks stdin is a tty

Hi,

I have a applypatch-msg hook that implements some policy for acceptable commit messages and reject non-conformant patches. It also is able to prompt me to override it's rejection. The prompting only happens when stdin is a tty (as determined by pythons sys.stdin.isatty())

For example this would reject the patch and cause am stop
 $ cat bad.patch | git am

And this would prompt me to allow the patch and am would proceed if I answer Y
  $ git am bad.patch
  Patch message invalid apply (Y/N)?

As of git 2.6 this has stopped working and stdin always fails the tty check. Here's a minimal reproduction

$ cat >.git/hooks/applypatch-msg <<EOF
#!/usr/bin/env python
import sys
if sys.stdin.isatty():
    print("TTY")
else:
    print("NOT")
EOF
$ chmod +x .git/hooks/applypatch-msg

$ git checkout -b temp origin/master^ $ git format-patch master -1

# With git 2.6
$ git am 0001-Git-2.6.patch
NOT
Applying: Git 2.6

# With git 2.5.3
TTY
Applying: Git 2.6

Thanks, Chris