Implement 'git reset --patch'

Implement 'git reset --patch'

This introduces a --patch mode for git-reset.  The basic case is

  git reset --patch -- [files...]

which acts as the opposite of 'git add --patch -- [files...]': it
offers hunks for *un*staging.  Advanced usage is

  git reset --patch <revision> -- [files...]

which offers hunks from the diff between the index and <revision> for
forward application to the index.  (That is, the basic case is just
<revision> = HEAD.)

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>