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