Clean up Git for Windows' branch thicket (#4417)

I tried to make future rebases easier by merging `main` into
v2.40.1.windows.1 in #4412, but that didn't work.

So let's clean up the branch thicket. with the result being
intentionally tree-same to `main`. This _will_ help with the upcoming
v2.41.0-rc0 rebase.

Author: dscho