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 use (repo, FALSE, FALSE). There may be some very old fs objects fail the strict object id test. But those objects are valid indeed.