Pick reviewers based on branch name

Change reviewer roulette to always pick the same reviewers for the same
branch name. We do this by:

1. Making the branch name 'canonical' across CE and EE by stripping a
   leading 'ce-' or 'ee-' and a trailing '-ce' or '-ee'. If people are
   following our branch naming guidelines, this should give the same
   branch name in both repos.
2. Converting the branch name to a stable integer by taking the integer
   form of its MD5.
3. Passing that integer as a seed to Ruby's `Random` class, which 'may
   be used to ensure repeatable sequences of pseudo-random numbers
   between different runs of the program' (from the Ruby documentation).

The upshot is that the same branch name (in CE and EE) should always
pick the same reviewers, and those should be evenly distributed across
the set of possible reviewers due to the use of MD5.

danger/roulette/Dangerfile

 # frozen_string_literal: true
 
+require 'digest/md5'
+
 MESSAGE = <<MARKDOWN
 ## Reviewer roulette
 
@@ -29,7 +31,9 @@ Please consider creating a merge request to
 for them.
 MARKDOWN
 
-def spin(team, project, category)
+def spin(team, project, category, branch_name)
+  rng = Random.new(Digest::MD5.hexdigest(branch_name).to_i(16))
+
   reviewers = team.select { |member| member.reviewer?(project, category) }
   traintainers = team.select { |member| member.traintainer?(project, category) }
   maintainers = team.select { |member| member.maintainer?(project, category) }
@@ -41,8 +45,8 @@ def spin(team, project, category)
   # https://gitlab.com/gitlab-org/gitlab-ce/issues/57653
 
   # Make traintainers have triple the chance to be picked as a reviewer
-  reviewer = (reviewers + traintainers + traintainers).sample
-  maintainer = maintainers.sample
+  reviewer = (reviewers + traintainers + traintainers).sample(random: rng)
+  maintainer = maintainers.sample(random: rng)
 
   "| #{helper.label_for_category(category)} | #{reviewer&.markdown_name} | #{maintainer&.markdown_name} |"
 end
@@ -68,6 +72,12 @@ categories = changes.keys - [:unknown]
 # CSS Clean up MRs are reviewed using a slightly different process, so we
 # disable the review roulette for such MRs.
 if changes.any? && !gitlab.mr_labels.include?('single codebase') && !gitlab.mr_labels.include?('CSS cleanup')
+  # Strip leading and trailing CE/EE markers
+  canonical_branch_name = gitlab
+                            .mr_json['source_branch']
+                            .gsub(/^[ce]e-/, '')
+                            .gsub(/-[ce]e$/, '')
+
   team =
     begin
       helper.project_team
@@ -82,7 +92,7 @@ if changes.any? && !gitlab.mr_labels.include?('single codebase') && !gitlab.mr_l
   project = helper.project_name
   unknown = changes.fetch(:unknown, [])
 
-  rows = categories.map { |category| spin(team, project, category) }
+  rows = categories.map { |category| spin(team, project, category, canonical_branch_name) }
 
   markdown(MESSAGE)
   markdown(CATEGORY_TABLE_HEADER + rows.join("\n")) unless rows.empty?