pkg/git: add SwitchCommit and GetCommit
pkg/git: add SwitchCommit and GetCommit

Will be useful for bisection logic.

Update #501

xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller