Expose initial_commit? helper to pre-commit hooks

This will allow these hooks to better handle odd situations that occur
only on an empty branch.

Author: sds

lib/overcommit/hook/pre_commit/base.rb

@@ -5,7 +5,7 @@ module Overcommit::Hook::PreCommit
   class Base < Overcommit::Hook::Base
     extend Forwardable
 
-    def_delegators :@context, :modified_lines_in_file, :amendment?
+    def_delegators :@context, :modified_lines_in_file, :amendment?, :initial_commit?
 
     private
 

lib/overcommit/hook_context/run_all.rb

@@ -32,6 +32,11 @@ def hook_script_name
       'pre-commit'
     end
 
+    def initial_commit?
+      return @initial_commit unless @initial_commit.nil?
+      @initial_commit = Overcommit::GitRepo.initial_commit?
+    end
+
     private
 
     def count_lines(file)