CMake: improve git check in GitUpdateSubmodules.cmake

[program--]

Note

Low priority, tracking for #875 (comment)

ngen/cmake/GitUpdateSubmodules.cmake

Lines 1 to 14 in 1cb6e41

	find_package(Git QUIET)
	if(EXISTS "${NGEN_ROOT_DIR}/.git")
	set(NGEN_HAS_GIT_DIR ON)
	execute_process(
	COMMAND ${GIT_EXECUTABLE} rev-parse HEAD
	WORKING_DIRECTORY ${NGEN_ROOT_DIR}
	OUTPUT_VARIABLE NGEN_GIT_COMMIT
	OUTPUT_STRIP_TRAILING_WHITESPACE
	)
	else()
	set(NGEN_HAS_GIT_DIR OFF)
	endif()

There might be edge cases associated with how we check that the current directory is a git repository.

Additionally, we probably should check for GIT_FOUND