Fix a problem in making link to commit in activities

When I push local change that is cutting a new branch having no commit, app display follow StackTrace.

<pre>
HTTP Status 500 - String index out of range: 40

type Exception report

message String index out of range: 40

description The server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.StringIndexOutOfBoundsException: String index out of range: 40
	java.lang.String.substring(String.java:1907)
	helper.html.activities$$anonfun$apply$1$$anonfun$apply$2.apply(activities.template.scala:80)
	helper.html.activities$$anonfun$apply$1$$anonfun$apply$2.apply(activities.template.scala:75)
	scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
...
</pre>

Fix that error by refference to [Ignore error in activity timeline caused by invalid data. · 3546a5d · takezoe/gitbucket](https://github.com/takezoe/gitbucket/commit/3546a5d392206a1e343de33cfaa5546b19957a3d "Ignore error in activity timeline caused by invalid data. · 3546a5d · takezoe/gitbucket")