Render not found when failing to look for a commit
A little improvement

1. Replace params key 'q' with 'extended_sha1'. A extended SHA1 syntax is explained in 'man gitrevisions'.
2. Change the placeholder of looking for commit.
3. Change the label of ref filter.
Merge pull request #4829 from hiroponz/render-not-found-when-failing-to-look-for-commit

Render not found when failing to look for commit