ThirdParty/Twisted, add update.sh script, readme

Comment update-common.sh on how to avoid failing option check
on windows.

README.kitware.md describes the changes for vtk to the
source Twisted. Correct the cmake source path.

ThirdParty/Twisted/CMakeLists.txt

 include(vtkModuleMacrosPython)
-vtk_module_python_package(${vtk-module} twisted CAN_USE_SYSTEM)
+vtk_module_python_package(${vtk-module} vtkTwisted/src/twisted CAN_USE_SYSTEM)

ThirdParty/Twisted/README.kitware.md

+# Twisted fork for vtk/ParaView
+
+This branch contains changes required to embed Twisted into vtk/ParaView. This
+includes choosing a sub-set without documentation, testing, and unused modules, and adding CMake build files.
+
+Use https://gitlab.kitware.com/third-party/twisted and update.sh to get new versions of Twisted. See [Updating][../UPDATING.md] for details.

ThirdParty/Twisted/update.sh

+#!/usr/bin/env bash
+set -e
+set -x
+shopt -s dotglob
+readonly name="Twisted"
+readonly ownership="Twisted Upstream <kwrobot@kitware.com>"
+readonly subtree="ThirdParty/$name/vtk$name"
+readonly repo="https://gitlab.kitware.com/third-party/twisted.git"
+readonly tag="for/vtk"
+readonly paths="
+NEWS
+LICENSE
+CONTRIBUTING
+README.rst
+.gitattributes
+src/twisted/*.py
+src/twisted/_threads
+src/twisted/application
+src/twisted/cred
+src/twisted/internet/*.py
+src/twisted/logger
+src/twisted/persisted
+src/twisted/plugins
+src/twisted/protocols
+src/twisted/python
+src/twisted/runner
+src/twisted/scripts
+src/twisted/spread
+src/twisted/tap
+src/twisted/web
+"
+# src/twisted/conch
+# src/twisted/enterprise
+# src/twisted/mail
+# src/twisted/names
+# src/twisted/news
+# src/twisted/pair
+# src/twisted/positioning
+# src/twisted/test
+# src/twisted/topfiles
+# src/twisted/trial
+# src/twisted/words
+extract_source () {
+    # Copy over the files from Git
+    git_archive
+}
+. "${BASH_SOURCE%/*}/../update-common.sh"

ThirdParty/imported.md

@@ -11,6 +11,7 @@
   * [mpi4py](mpi4py/update.sh)
   * [png](png/update.sh)
   * [tiff](tiff/update.sh)
+  * [Twisted](Twisted/update.sh)
   * [xdmf3](xdmf3/update.sh)
   * [zfp](zfp/update.sh)
   * [zlib](zlib/update.sh)

ThirdParty/update-common.sh

@@ -155,6 +155,8 @@ popd
 if [ -n "$basehash" ]; then
     git merge --log -s recursive "-Xsubtree=$subtree/" --no-commit "upstream-$name"
 else
+    # Note: on Windows this will likely fail. Edit the script to skip the check
+    # and use the flag.
     unrelated_histories_flag=""
     if git merge --help | grep -q -e allow-unrelated-histories; then
         unrelated_histories_flag="--allow-unrelated-histories "