Fix incorrect comparison of reflog syntax

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Reflog hash is repeatable!

map should not be used

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Drop unused methods

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Use libgit2 to read reflog

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Fixed issue #2008: Wrong reflog if hash repeated