[rubygems/rubygems] Vendor a pure ruby implementation of SHA1

This allows `Source::Git` to no longer load the `digest` gem as it is causing
issues on Ruby 3.1.

https://github.com/rubygems/rubygems/pull/4989/commits/c19a9f2ff7

lib/bundler/digest.rb

@@ -0,0 +1,71 @@
+# frozen_string_literal: true
+
+# This code was extracted from https://github.com/Solistra/ruby-digest which is under public domain
+module Bundler
+  module Digest
+    # The initial constant values for the 32-bit constant words A, B, C, D, and
+    # E, respectively.
+    SHA1_WORDS = [0x67452301, 0xEFCDAB89, 0x98BADCFE, 0x10325476, 0xC3D2E1F0].freeze
+
+    # The 8-bit field used for bitwise `AND` masking. Defaults to `0xFFFFFFFF`.
+    SHA1_MASK = 0xFFFFFFFF
+
+    class << self
+      def sha1(string)
+        unless string.is_a?(String)
+          raise TypeError, "can't convert #{string.class.inspect} into String"
+        end
+
+        buffer = string.b
+
+        words = SHA1_WORDS.dup
+        generate_split_buffer(buffer) do |chunk|
+          w = []
+          chunk.each_slice(4) do |a, b, c, d|
+            w << (((a << 8 | b) << 8 | c) << 8 | d)
+          end
+          a, b, c, d, e = *words
+          (16..79).each do |i|
+            w[i] = SHA1_MASK & rotate((w[i-3] ^ w[i-8] ^ w[i-14] ^ w[i-16]), 1)
+          end
+          0.upto(79) do |i|
+            case i
+            when  0..19
+              f = ((b & c) | (~b & d))
+              k = 0x5A827999
+            when 20..39
+              f = (b ^ c ^ d)
+              k = 0x6ED9EBA1
+            when 40..59
+              f = ((b & c) | (b & d) | (c & d))
+              k = 0x8F1BBCDC
+            when 60..79
+              f = (b ^ c ^ d)
+              k = 0xCA62C1D6
+            end
+            t = SHA1_MASK & (SHA1_MASK & rotate(a, 5) + f + e + k + w[i])
+            a, b, c, d, e = t, a, SHA1_MASK & rotate(b, 30), c, d # rubocop:disable Style/ParallelAssignment
+          end
+          mutated = [a, b, c, d, e]
+          words.map!.with_index {|word, index| SHA1_MASK & (word + mutated[index]) }
+        end
+
+        words.pack("N*").unpack("H*").first
+      end
+
+      private
+
+      def generate_split_buffer(string, &block)
+        size   = string.bytesize * 8
+        buffer = string.bytes << 128
+        buffer << 0 while buffer.size % 64 != 56
+        [size].pack("Q").bytes.reverse_each {|b| buffer << b }
+        buffer.each_slice(64, &block)
+      end
+
+      def rotate(value, spaces)
+        value << spaces | value >> (32 - spaces)
+      end
+    end
+  end
+end