Remove #process_commit_data default sha value

As with the previous commit, this default value was relevant when this
method was also used to process git log output. It no longer is, and its
only caller passes the sha value, so we can remove the default.

Signed-off-by: Simon Coffey <simon.coffey@futurelearn.com>

lib/git/lib.rb

@@ -224,7 +224,7 @@ def commit_data(sha)
       process_commit_data(cdata, sha)
     end
 
-    def process_commit_data(data, sha = nil)
+    def process_commit_data(data, sha)
       hsh = {
         'sha'    => sha,
         'parent' => []