package/git: make _BUG_ condition more clear

As pointed by Peter combined condition of the 2 gcc bugs is potentially
wrong, but as Thomas pointed in this case it's not harmful. Let's fix it
anyway since it's basically wrong even it doesn't cause harm.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>