[fix] Attempt to fix incorrect `pod list [DAYS]'

The command can picked a commit that was in a
branch and presented an overestimated number of new pods
as the branch may include the pods added to master
during its life time

The issues is recognizable by the following pattern
`$pod list 2' => shows 20 new pods
`$pod list 3' => shows a correct low number of new pods

lib/cocoapods/command/list.rb

@@ -39,7 +39,7 @@ module Pod
       end
 
       def commit_from_days_ago (days)
-        Dir.chdir(dir) { git("rev-list -n1 --before=\"#{days} day ago\" master") }
+        Dir.chdir(dir) { git("rev-list -n1 --before=\"#{days} day ago\" --first-parent master") }
       end
 
       def spec_names_from_commit (commit)
@@ -52,6 +52,9 @@ module Pod
 
         result = dir_list.split("\n").uniq
         result.delete('')
+        puts
+        puts commit.white
+        puts result.join(', ').magenta
         result
       end