To have compatibility with GitHub/GitHubEnterprise

when archive downloading with sha1, even if supplied part of sha1,
GH/GHE gives a filename with full(long) sha1.

This patch make GitBucket to be compatible with GH/GHE behaviour.

You can check that difference by following url.
I upload same repo as in GitHuB to GitBucket demo site.
 
GitHub:  
https://github.com/MunGell/awesome-for-beginners/archive/fc7d067.tar.gz
GitBucket:
http://gitbucket.herokuapp.com/root/awesome-for-beginners/archive/fc7d067.tar.gz

GH returns,
awesome-for-beginners-fc7d067cb13559f248bb362253ff2fa3b2617aba.tar.gz
Otherwise GitBucket returns,
awesome-for-beginners-fc7d067.tar.gz

src/main/scala/gitbucket/core/controller/RepositoryViewerController.scala

@@ -671,12 +671,14 @@ trait RepositoryViewerControllerBase extends ControllerBase {
   private def archiveRepository(name: String, suffix: String, repository: RepositoryService.RepositoryInfo): Unit = {
     val revision = name.stripSuffix(suffix)
     
-    val filename = repository.name + "-" +
-      (if(revision.length == 40) revision.substring(0, 10) else revision).replace('/', '-') + suffix
-
     using(Git.open(getRepositoryDir(repository.owner, repository.name))){ git =>
       val revCommit = JGitUtil.getRevCommitFromId(git, git.getRepository.resolve(revision))
       
+      val sha1 = git.getRepository.resolve(revision).getName()
+      
+      val filename = repository.name + "-" +
+        (if(sha1.startsWith(revision)) sha1 else revision).replace('/','-') + suffix
+
       contentType = "application/octet-stream"
       response.setHeader("Content-Disposition", s"attachment; filename=${filename}")
       response.setBufferSize(1024 * 1024);