Thanks for your encouragement, Matt. Unfortunately, even when following your recipe, I still get:
cedekker@2-lkeb-17-nde1 MINGW64 /f/X/Src/I/ITK (master)
$ git checkout -b googletest-update
Switched to a new branch ‘googletest-update’
cedekker@2-lkeb-17-nde1 MINGW64 /f/X/Src/I/ITK (googletest-update)
Created upstream snapshot branch ‘upstream-googletest’.
Performing inexact rename detection: 100% (666708/666708), done.
CONFLICT (file location): googletest/src/gtest-matchers.cc added in upstream-googletest inside a directory that was renamed in HEAD, suggesting it should perhaps be moved to Modules/ThirdParty/GoogleTest/src/itkgoogletest/googletest/src/gtest-matchers.cc.
CONFLICT (content): Merge conflict in CMakeLists.txt
Automatic merge failed; fix conflicts and then commit the result.
error: The branch ‘upstream-googletest’ is not fully merged.
If you are sure you want to delete it, run ‘git branch -D upstream-googletest’.
cedekker@2-lkeb-17-nde1 MINGW64 /f/X/Src/I/ITK (googletest-update|MERGING)
Do you have a clue?