Implement 'git checkout --patch'

This introduces a --patch mode for git-checkout.  In the index usage

  git checkout --patch -- [files...]

it lets the user discard edits from the <files> at the granularity of
hunks (by selecting hunks from 'git diff' and then reverse applying
them to the worktree).

We also accept a revision argument.  In the case

  git checkout --patch HEAD -- [files...]

we offer hunks from the difference between HEAD and the worktree, and
reverse applies them to both index and worktree, allowing you to
discard staged changes completely.  In the non-HEAD usage

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

it offers hunks from the difference between the worktree and
<revision>.  The chosen hunks are then applied to both index and
worktree.

The application to worktree and index is done "atomically" in the
sense that we first check if the patch applies to the index (it should
always apply to the worktree).  If it does not, we give the user a
choice to either abort or apply to the worktree anyway.

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

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