Refactor ExtractsRef#extracts_ref to be clearer

This method has grown a lot over time. One thing that makes the control
flow quite confusing is that we don't return early, but always do
something with the `pair` variable at the end of the method.

Splitting this method into two methods means that we can return early
from the core logic, making that part easier to understand, while doing
our final string manipulation in the calling method.

lib/extracts_ref.rb

@@ -40,50 +40,11 @@ module ExtractsRef
   # Returns an Array where the first value is the tree-ish and the second is the
   # path
   def extract_ref(id)
-    pair = ['', '']
-
-    return pair unless repository_container
-
-    if id =~ /^(\h{40})(.+)/
-      # If the ref appears to be a SHA, we're done, just split the string
-      pair = $~.captures
-    elsif id.exclude?('/')
-      # If the ID contains no slash, we must have a ref and no path, so
-      # we can skip the Redis calls below
-      pair = [id, '']
-    else
-      # Otherwise, attempt to detect the ref using a list of the repository_container's
-      # branches and tags
-
-      # Append a trailing slash if we only get a ref and no file path
-      unless id.ends_with?('/')
-        id = [id, '/'].join
-      end
-
-      first_path_segment, rest = id.split('/', 2)
-
-      if use_first_path_segment?(first_path_segment)
-        pair = [first_path_segment, rest]
-      else
-        valid_refs = ref_names.select { |v| id.start_with?("#{v}/") }
-
-        if valid_refs.empty?
-          # No exact ref match, so just try our best
-          pair = id.match(%r{([^/]+)(.*)}).captures
-        else
-          # There is a distinct possibility that multiple refs prefix the ID.
-          # Use the longest match to maximize the chance that we have the
-          # right ref.
-          best_match = valid_refs.max_by(&:length)
-          # Partition the string into the ref and the path, ignoring the empty first value
-          pair = id.partition(best_match)[1..-1]
-        end
-      end
-    end
+    pair = extract_raw_ref(id)
 
     [
       pair[0].strip,
-      pair[1].gsub(%r{^/|/$}, '') # Remove leading and trailing slashes from path
+      pair[1].delete_prefix('/').delete_suffix('/')
     ]
   end
 
@@ -117,6 +78,38 @@ module ExtractsRef
 
   private
 
+  def extract_raw_ref(id)
+    return ['', ''] unless repository_container
+
+    # If the ref appears to be a SHA, we're done, just split the string
+    return $~.captures if id =~ /^(\h{40})(.+)/
+
+    # No slash means we must have a ref and no path
+    return [id, ''] unless id.include?('/')
+
+    # Otherwise, attempt to detect the ref using a list of the
+    # repository_container's branches and tags
+
+    # Append a trailing slash if we only get a ref and no file path
+    id = [id, '/'].join unless id.ends_with?('/')
+    first_path_segment, rest = id.split('/', 2)
+
+    return [first_path_segment, rest] if use_first_path_segment?(first_path_segment)
+
+    valid_refs = ref_names.select { |v| id.start_with?("#{v}/") }
+
+    # No exact ref match, so just try our best
+    return id.match(%r{([^/]+)(.*)}).captures if valid_refs.empty?
+
+    # There is a distinct possibility that multiple refs prefix the ID.
+    # Use the longest match to maximize the chance that we have the
+    # right ref.
+    best_match = valid_refs.max_by(&:length)
+
+    # Partition the string into the ref and the path, ignoring the empty first value
+    id.partition(best_match)[1..-1]
+  end
+
   def use_first_path_segment?(ref)
     return false unless ::Feature.enabled?(:extracts_path_optimization)
     return false unless repository_container