fix bashism in git check-ref-format call

'system' spawns a default shell, and on many systems (e.g. Debian
starting with 6.0 / Squeezy) the default shell isn't /bin/bash but
/bin/dash instead. So let's only use POSIX features and drop the
bash-specific '>&' shortcut.
Merge remote-tracking branch 'remotes/origin/bashism' into mailnotification