Creating a branch a/b/c when a/b exists fails complaining that a ref 'a/b' just exists.

This is probably due to the fact that when creating a/b a directory 'a' is created inside 
'refs/heads' and a file 'b' is created under the a directory, avoiding the subsequent possibility 
to create a folder 'b' in the same directory that is needed to create the ref for 'a/b/c'

The solution seems to append to the file an extension to disambiguate the file from the folder.

[System Info]
git version:
git version 2.27.0.windows.1
cpu: x86_64
built from commit: 907ab1011dce9112700498e034b974ba60f8b407
sizeof-long: 4
sizeof-size_t: 8
uname: Windows 10.0 18363 
compiler info: gnuc: 10.1
libc info: no libc information available


[Enabled Hooks]