Fix Repository#has_ambiguous_refs? regex

This regex intended to check if any refs that do not contain a slash
were a prefix of a ref containing the slash, plus the slash. That is:
`abc` is a prefix of `abc/def`, but not of `abcd/ef`.

However, the regex grouping was incorrect, and we were creating a regex
like `%r{^abc|def/}`, which doesn't contain parentheses, and so would
actually match plain `abc` (it wouldn't match `def/`).

app/models/repository.rb

@@ -214,7 +214,7 @@ class Repository
     return false if with_slash.empty?
 
     prefixes = no_slash.map { |ref| Regexp.escape(ref) }.join('|')
-    prefix_regex = %r{^#{prefixes}/}
+    prefix_regex = %r{^(#{prefixes})/}
 
     with_slash.any? do |ref|
       prefix_regex.match?(ref)

spec/models/repository_spec.rb

@@ -1263,6 +1263,7 @@ RSpec.describe Repository do
       %w(a b c/z) | %w(c d) | true
       %w(a/b/z) | %w(a/b) | false # we only consider refs ambiguous before the first slash
       %w(a/b/z) | %w(a/b a) | true
+      %w(ab) | %w(abc/d a b) | false
     end
 
     with_them do