Make HookContext::PreCommit#initial_commit? public

Since it's being forwarded to, in Ruby 2.4 it throws a warning.

Author: thegranddesign

lib/overcommit/hook_context/pre_commit.rb

@@ -126,6 +126,12 @@ def modified_lines_in_file(file)
       @modified_lines[file]
     end
 
+    # Returns whether the current git branch is empty (has no commits).
+    def initial_commit?
+      return @initial_commit unless @initial_commit.nil?
+      @initial_commit = Overcommit::GitRepo.initial_commit?
+    end
+
     private
 
     # Clears the working tree so that the stash can be applied.
@@ -170,12 +176,6 @@ def any_changes?
       modified_files.any?
     end
 
-    # Returns whether the current git branch is empty (has no commits).
-    def initial_commit?
-      return @initial_commit unless @initial_commit.nil?
-      @initial_commit = Overcommit::GitRepo.initial_commit?
-    end
-
     # Stores the modification times for all modified files to make it appear like
     # they never changed.
     #