FetchContent and ExternalProject fail when SOURCE_DIR changes

I open this issue in response to https://discourse.cmake.org/t/error-pathspec-did-not-match-any-file-s-known-to-git. Changing SOURCE_DIR for fetch content or external project results in fatal error for git. The only way to recover from this state is by manually deleting files inside the build directory.

a minimal example:

cmake_minimum_required( VERSION 3.16 )
project(MyProj VERSION 0 LANGUAGES NONE)
include(FetchContent)
FetchContent_Declare(myscript
  GIT_REPOSITORY git@gist.github.com:76c2097a0b6a74c870f6b50c2cf1f470.git
  SOURCE_DIR foo)

# FetchContent_GetProperties(myscript)
# if( NOT cmakebuild_POPULATED )
# 	FetchContent_Populate(myscript)
# endif()
FetchContent_MakeAvailable(myscript)

results in error: error: pathspec did not match any file(s) known to git

another example:

cmake_minimum_required( VERSION 3.16 )
project(MyProj VERSION 0 LANGUAGES NONE)
include(ExternalProject)
ExternalProject_Add(myscript
	GIT_REPOSITORY 	git@gist.github.com:76c2097a0b6a74c870f6b50c2cf1f470.git
	SOURCE_DIR        ${CMAKE_CURRENT_LIST_DIR}/foo
	CONFIGURE_COMMAND ""
	BUILD_COMMAND     ""
	INSTALL_COMMAND   ""
)

results in error: Failed to get the hash for HEAD

my setup: CMake 3.19, Gnu Makefile generator, Linux 64b