git.is_git: optimize detection: does .git exist?
git.is_git: optimize detection: does .git exist?

returns True if a [path , .git] exists

This is ~8 times faster on windows than calling:
git rev-parse --show-toplevel

Change-Id: Ibe11c7bb7b3804ad27b34a1af392675b4982b118
Reviewed-on: http://gerrit.aldebaran.lan/30269
Tested-by: gerrit
Reviewed-by: vbarbaresi <371b46c96c99af52f4f920034e4fcb63ece5bdb5@aldebaran-robotics.com>

aldebaran/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild,aldebaran/qibuild,dmerejkowsky/qibuild,dmerejkowsky/qibuild