Fix missing object exception during outdated detecting
Fix missing object exception during outdated detecting

Problem: While checking whether a comment thread is outdated,
MissingObjectException might occur if the thread is on a missing git
object.

Solution:
* Consider such kind of thread as outdated one to avoid the exception.
* Log a warning message.

yona-projects/yona,doortts/fork-yobi,ihoneymon/yobi,oolso/yobi,naver/yobi,brainagenet/yobi,doortts/forked-for-history,yona-projects/yona,doortts/fork-yobi,bloodybear/yona,brainagenet/yobi,ahb0327/yobi,naver/yobi,violetag/demo,ahb0327/yobi,naver/yobi,bloodybear/yona,ihoneymon/yobi,Limseunghwan/oss,Limseunghwan/oss,ihoneymon/yobi,bloodybear/yona,oolso/yobi,doortts/fork-yobi,doortts/forked-for-history,oolso/yobi,violetag/demo,Limseunghwan/oss,doortts/forked-for-history,yona-projects/yona,yona-projects/yona,bloodybear/yona,doortts/fork-yobi,brainagenet/yobi,ahb0327/yobi