tortoisehg: fix build

tortoisehg intentionally wants to back-date the Mercurial build,
and some of the features we've added to the Mercurial build won't work
correctly under hg 5.6.

pkgs/applications/version-management/mercurial/default.nix

@@ -1,4 +1,5 @@
 { lib, stdenv, fetchurl, fetchpatch, python3Packages, makeWrapper, gettext
+, re2Support ? true
 , rustSupport ? stdenv.hostPlatform.isLinux, rustPlatform
 , guiSupport ? false, tk ? null
 , ApplicationServices
@@ -41,7 +42,7 @@ in python3Packages.buildPythonApplication rec {
   } else null;
   cargoRoot = if rustSupport then "rust" else null;
 
-  propagatedBuildInputs = [ fb-re2 ];
+  propagatedBuildInputs = lib.optional re2Support fb-re2;
   nativeBuildInputs = [ makeWrapper gettext ]
     ++ lib.optionals rustSupport (with rustPlatform; [
          cargoSetupHook

pkgs/applications/version-management/tortoisehg/default.nix

@@ -9,12 +9,16 @@ let
     sha256 = "031bafj88wggpvw0lgvl0djhlbhs9nls9vzwvni8yn0m0bgzc9gr";
   };
 
-  tortoiseMercurial = mercurial.overridePythonAttrs (old: rec {
+  tortoiseMercurial = (mercurial.override {
+    rustSupport = false;
+    re2Support = lib.versionAtLeast tortoisehgSrc.meta.version "5.8";
+  }).overridePythonAttrs (old: rec {
     inherit (tortoisehgSrc.meta) version;
     src = fetchurl {
       url = "https://mercurial-scm.org/release/mercurial-${version}.tar.gz";
       sha256 = "1hk2y30zzdnlv8f71kabvh0xi9c7qhp28ksh20vpd0r712sv79yz";
     };
+    patches = [];
   });
 
 in python3Packages.buildPythonApplication {