Documentation/git-clean: fix description for range

The descriptions of "select by numbers" section for interactive
git-clean are borrowed from git-add, and one sentence should be
replaced.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
git-clean: implement partial matching for selection

Document for interactive git-clean says: "You also could say `c` or
`clean` above as long as the choice is unique". But it's not true,
because only hotkey `c` and full match (`clean`) could work.

Implement partial matching via find_unique function to make the
document right.

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>