reset: support "--mixed --intent-to-add" mode

reset: support "--mixed --intent-to-add" mode

When --mixed is used, entries could be removed from index if the
target ref does not have them. When "reset" is used in preparation for
commit spliting (in a dirty worktree), it could be hard to track what
files to be added back. The new option --intent-to-add simplifies it
by marking all removed files intent-to-add.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>