scripts/git.sh: fix bashisms

voodoo@hestia:/opt/github/stable-kernel$ ./checkbashisms scripts/git.sh
script scripts/git.sh does not appear to be a /bin/sh script
possible bashism in scripts/git.sh line 35 (should be >word 2>&1):
        git tag | grep v${KERNEL_TAG} &>/dev/null || git_kernel_stable
possible bashism in scripts/git.sh line 115 (should be >word 2>&1):
        git branch -D master &>/dev/null || true
possible bashism in scripts/git.sh line 118 (should be >word 2>&1):
        git branch -D tmp-master &>/dev/null || true
possible bashism in scripts/git.sh line 122 (should be >word 2>&1):
        git tag | grep v${KERNEL_TAG} | grep -v rc &>/dev/null || git_kernel_torvalds
possible bashism in scripts/git.sh line 128 (should be >word 2>&1):
        git branch -D v${KERNEL_TAG}-${BUILD} &>/dev/null || true
possible bashism in scripts/git.sh line 145 (should be '.', not 'source'):
source ${DIR}/version.sh
possible bashism in scripts/git.sh line 146 (should be '.', not 'source'):
source ${DIR}/system.sh
possible bashism in scripts/git.sh line 158 (should be >word 2>&1):
        git branch | grep "*" | grep master &>/dev/null && ON_MASTER=1

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>