do_read_index() mmaps the index, or tries to die with an error message
on failure. It should call xmmap_gently(), which returns MAP_FAILED,
rather than xmmap(), which dies with its own error message.
An easy way to cause this mmap to fail is by setting $GIT_INDEX_FILE to
a path to a directory and then invoking any command that reads from the
index.
Signed-off-by: Varun Naik <vcnaik94@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>