Merge branch 'buttons-size' into 'master'

Fix size of disabled 'New file' button

Before:

![Screen_Shot_2015-12-09_at_13.52.30](/uploads/d3d5dce9b5fcb5a4292177acba323d17/Screen_Shot_2015-12-09_at_13.52.30.png)

After:

![Screen_Shot_2015-12-09_at_13.52.20](/uploads/a813fb817cfd4bff6b17fad34e79f3b1/Screen_Shot_2015-12-09_at_13.52.20.png)


See merge request !2040

app/views/projects/tree/_tree_header.html.haml

@@ -32,5 +32,5 @@
               New directory
   - elsif !on_top_of_branch?
     %li
-      %span.btn.add-to-tree.disabled.has_tooltip{title: "You can only add files when you are on a branch.", data: {container: 'body'}}
+      %span.btn.btn-sm.add-to-tree.disabled.has_tooltip{title: "You can only add files when you are on a branch.", data: {container: 'body'}}
         = icon('plus')