Ensure display file content of specified commit

fix this error

```
HTTP ERROR 500

Problem accessing /root/gitbucket/blob/71751ae4bc7a4a2fedcfb3a6b4a19d62e90642d1/src/main/scala/service/IssuesService.scala. Reason:

    Server Error

Caused by:

java.lang.NullPointerException
	at org.eclipse.jgit.lib.ObjectIdOwnerMap.get(ObjectIdOwnerMap.java:131)
	at org.eclipse.jgit.revwalk.RevWalk.parseAny(RevWalk.java:807)
	at util.JGitUtil$.getRevCommitFromId(JGitUtil.scala:144)
	at app.RepositoryViewerControllerBase$$anonfun$7$$anonfun$apply$8$$anonfun$apply$9.apply(RepositoryViewerController.scala:79)
	at app.RepositoryViewerControllerBase$$anonfun$7$$anonfun$apply$8$$anonfun$apply$9.apply(RepositoryViewerController.scala:78)
	at util.JGitUtil$.withGit(JGitUtil.scala:129)
	at util.JGitUtil$.withGit(JGitUtil.scala:121)
	at app.RepositoryViewerControllerBase$$anonfun$7$$anonfun$apply$8.apply(RepositoryViewerController.scala:78)
	at app.RepositoryViewerControllerBase$$anonfun$7$$anonfun$apply$8.apply(RepositoryViewerController.scala:74)
```

I think splitPath should be split by / when specified commit.