pkg/git: improve Poll
pkg/git: improve Poll

Support changing repo/branch and force pushes.

xairy/syzkaller,xairy/syzkaller,google/syzkaller,hyz0906/syzkaller,hyz0906/syzkaller,zoulasc/syzkaller,xairy/syzkaller,google/syzkaller,hyz0906/syzkaller,dotweiba/syzkaller,xairy/syzkaller,google/syzkaller,zoulasc/syzkaller,dotweiba/syzkaller,hyz0906/syzkaller,zoulasc/syzkaller,zoulasc/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,dotweiba/syzkaller,google/syzkaller,hyz0906/syzkaller,xairy/syzkaller,dotweiba/syzkaller,xairy/syzkaller