[CodeExtractor] checkout before build_prune_script

Prior, when doing `build_prune_script`, the branch was left at what ever
state the previous step had left it at.  However, this caused and issue
when running `build_prune_script` as it was attempting to find the
history for files that were just deleted in the `extract_branch` step
(the branch that will be pushed to the `upstream_repo` that "deletes
code").

This step ensures a `git checkout #{source_branch}` prior to building
the script to ensure the files and their previous versions still exist.

And additional tweak was also added to split up the initialization of
the `file_and_ancestors` variable to make it easier to debug.

Author: NickLaMuro

lib/code_extractor.rb

@@ -287,6 +287,7 @@ def build_prune_script extractions
       prune_mvs       = []
 
       Dir.chdir git_dir do
+        `git checkout #{source_branch}`
         extractions.each do |file_or_dir|
           if Dir.exist? file_or_dir
             files = Dir["#{file_or_dir}/**/*"]
@@ -295,7 +296,8 @@ def build_prune_script extractions
           end
 
           files.each do |extraction_file|
-            file_and_ancestors = `#{git_log_follow} -- #{extraction_file}`.split("\n").uniq
+            raw_file_history   = `#{git_log_follow} -- #{extraction_file}`
+            file_and_ancestors = raw_file_history.split("\n").uniq
 
             file_and_ancestors.reject! { |file| file.length == 0 }