git-add -i: allow multiple selection in patch subcommand

git-add -i: allow multiple selection in patch subcommand

This allows more than one files from the list to be chosen from
the patch subcommand instead of going through the file one by
one.

This also updates the "list-and-choose" UI for usability.  When
the prompt ends with ">>", if you type '*' to choose all
choices, the prompt immediately returns the choice without
requiring an extra empty line to confirm the selection.

Signed-off-by: Junio C Hamano <gitster@pobox.com>