simplify the detection of whether the source folder is checked out from a local git repo

set(temp_git_branch "(unknown)")

set(temp_git_describe "(unknown)")

set(temp_git_info "false")

if(GIT_FOUND)

  execute_process(COMMAND ${GIT_EXECUTABLE} describe HEAD

    RESULT_VARIABLE temp_in_git_checkout

    ERROR_QUIET

    OUTPUT_STRIP_TRAILING_WHITESPACE)

  if(temp_in_git_checkout EQUAL 0)

if(GIT_FOUND AND EXISTS ${CMAKE_SOURCE_DIR}/.git)

  set(temp_git_info "true")

  execute_process(COMMAND ${GIT_EXECUTABLE} rev-parse HEAD

    OUTPUT_VARIABLE temp_git_commit

    ERROR_QUIET

    OUTPUT_STRIP_TRAILING_WHITESPACE)

endif()

endif()

set(temp "${temp}const bool LAMMPS_NS::LAMMPS::has_git_info = ${temp_git_info};\n")

set(temp "${temp}const char LAMMPS_NS::LAMMPS::git_commit[] = \"${temp_git_commit}\";\n")