add --show-commit option for seaf-fsck

Except add --show-commit option, I also did follow modification:
(1) When fsck fetch master commit failed, it will try to fetch latest available commit from local disk.
(2) When update branch to latest available commit, using seaf_branch_manager_add_branch interface not seaf_branch_manager_update_branch, it will keep db consistent when the master branch row miss or commit object corrupted.
Should add a comment about why you use add_branch instead of update_branch.