Accept a valid ref for issue show

For example, now we support `/gitlab issue show #1`. Where the # used
to trip the regex.

lib/gitlab/chat_commands/issue_show.rb

@@ -2,7 +2,7 @@ module Gitlab
   module ChatCommands
     class IssueShow < IssueCommand
       def self.match(text)
-        /\Aissue\s+show\s+(?<iid>\d+)/.match(text)
+        /\Aissue\s+show\s+#{Issue.reference_prefix}?(?<iid>\d+)/.match(text)
       end
 
       def self.help_message

spec/lib/gitlab/chat_commands/issue_show_spec.rb

@@ -19,6 +19,14 @@ describe Gitlab::ChatCommands::IssueShow, service: true do
       it 'returns the issue' do
         expect(subject.iid).to be issue.iid
       end
+
+      context 'when its reference is given' do
+        let(:regex_match) { described_class.match("issue show #{issue.to_reference}") }
+
+        it 'shows the issue' do
+          expect(subject.iid).to be issue.iid
+        end
+      end
     end
 
     context 'the issue does not exist' do