Add "re-write rules" for bors comments

This gives us a fairly easy way to close specific tickets in favor of a
more general one, without having to edit the corresponding bors-comments.

This is done by writing e.g. "(Instance of #2)" for issue #1, if you
want re-annotate failures from #1 as really being from #2.

scripts/bors-stats.rb

@@ -68,6 +68,7 @@ def list()
       puts c.pretty(showDetails = options[:details])
     end
 
+    # TODO: De-duplicate
     tm = fetch_gh_ticket_titlemap options
     nTot = comments[:filtered].count
     nExcluded = comments[:unfiltered].length - nTot
@@ -346,6 +347,11 @@ def failure_rate(n, nTot)
 def fetch_comments_with_options(options)
   comments = fetch_comments(target = options[:count], nil, options["force-refetch"]).sort_by { |x| event_date x }
 
+  # Fetching the title map here ourselves is suboptimal, but
+  # since we have on-disk caching is should be ok.
+  tm = fetch_gh_ticket_titlemap options
+  rewrite_tags(comments, tm)
+
   if options[:search] then
     comments = comments.filter {|x| x.bodyText.include? options[:search] }
   end
@@ -638,6 +644,27 @@ def debug_trace(msg)
   end
 end
 
+# Re-writes the tags of the provided comments list in-place,
+# according to user-provided annotations in the issue titles.
+#
+# Annotations:
+# "(Instance of #2) ..." - replace tag with #2
+def rewrite_tags(comments, title_map)
+  rewrite_map = {}
+  title_map.each do |k,v|
+    new_tag = v["title"].scan(/^\(Instance of (#[\d]+)\)/).to_a.map { |x| x[0] }.first
+    if new_tag then
+      rewrite_map[k] = new_tag
+    end
+  end
+  comments.each do |c|
+    c.tags = c.tags.map do |t|
+      t2 = rewrite_map[t]
+      if t2 then t2 else t end
+    end
+  end
+end
+
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