PullRequest: Shorten merged branch on top of one-line descriptions
PullRequest: Shorten merged branch on top of one-line descriptions

When Yobi generates a merge commit message, shorten the refname to the
merged branch on top of one-line descriptions.

Before:

    * refs/heads/docs/relnotes-0.8.0:
      Do not use MVCC
      Update the release notes for v0.8.0

Now:

    * docs/relnotes-0.8.0:
      Do not use MVCC
      Update the release notes for v0.8.0

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