Move blob_snippet to gitlab_git gem

lib/gitlab/blob_snippet.rb

-module Gitlab
-  class BlobSnippet
-    include Linguist::BlobHelper
-
-    attr_accessor :project
-    attr_accessor :tree
-    attr_accessor :lines
-    attr_accessor :filename
-    attr_accessor :startline
-    
-    def initialize(project, tree, lines, startline, filename)
-      @project, @tree, @lines, @startline, @filename = project, tree, lines, startline, filename
-    end
-    
-    def data
-      lines.join("\n")
-    end
-    
-    def name
-      filename
-    end
-    
-    def size
-      data.length
-    end
-    
-    def mode
-      nil
-    end
-    
-  end
-end
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