ruby_2_7: init at 2.7.0

Release notes: https://www.ruby-lang.org/en/news/2019/12/25/ruby-2-7-0-released/

pkgs/development/interpreters/ruby/default.nix

@@ -27,6 +27,7 @@ let
     ver = version;
     tag = ver.gitTag;
     atLeast25 = lib.versionAtLeast ver.majMin "2.5";
+    atLeast27 = lib.versionAtLeast ver.majMin "2.7";
     baseruby = self.override {
       useRailsExpress = false;
       docSupport = false;
@@ -116,6 +117,12 @@ let
           cp ${config}/config.sub tool/
         '';
 
+        # Force the revision.h generation. Somehow `revision.tmp` is an empty
+        # file and because we don't add `git` to buildInputs, hence the check is
+        # always true.
+        # https://github.com/ruby/ruby/commit/97a5af62a318fcd93a4e5e4428d576c0280ddbae
+        buildFlags = lib.optionals atLeast27 [ "REVISION_LATEST=0" ];
+
         configureFlags = ["--enable-shared" "--enable-pthread" "--with-soname=ruby_${tag}"]
           ++ op useRailsExpress "--with-baseruby=${baseruby}/bin/ruby"
           ++ op (!docSupport) "--disable-install-doc"
@@ -246,4 +253,12 @@ in {
       git = "0pay6ic22ag3bnvxffhgwp7z6clkd0p93944a1l4lvc5hxc8v77j";
     };
   };
+
+  ruby_2_7 = generic {
+    version = rubyVersion "2" "7" "0" "";
+    sha256 = {
+      src = "1glc3zpnih6h8mrgfcak0aa7cgmi4zyvxfyi6y2brwg2nn9sm6cc";
+      git = "11iz64k95czs273mb10195d1j75mmbcgddfdx1vay5876ffw81dq";
+    };
+  };
 }