Assertion failed on git-svn checkout

I have the exact same issue on Windows 10 and sadly no real solution for you. But maybe it helps in the meantime to install the prior version 2.26.2. Link

In my case it "solved" the problem until a proper solution is found.

Update (2020-07-21):

Currently a release candidate for version 2.28.0 exists where this problem was fixed. Link

This is a known bug in the 2.27.0 Release and is Fixed in Higher Versions See https://github.com/git-for-windows/git/issues/2649

Until then you can roll back to 2.26.2 if you are unwilling to take a prerelease (it is fixed as of the July 7th 2020 Prerelease).