Speed up the "Project Network Graph" cucumber feature

This change involves stubbing out the call to `Grit::Commit.find_all`
and limiting the number of commits to 10 vs. the standard of 650 used by
`Gitlab::GraphCommit.to_graph`.

Prior to this change, this single feature required almost 3 minutes of
running time and over 2 GB of memory on my machine. Now it takes less
than 3 seconds.

features/support/env.rb

@@ -44,3 +44,5 @@ require 'headless'
 
 headless = Headless.new
 headless.start
+
+require 'cucumber/rspec/doubles'