revert change to ignore_skip_worktree in git checkout and git restore

[derrickstolee]

This reverts #658 and essentially reopens microsoft/VFSForGit#1812. Resolves #673.

The issue is that by overriding the ignore_skip_wortree value (normally set by --ignore-skip-worktree) we then force Git to hydrate the entire working directory in a git restore .. This leads to some interesting trade-offs:

1. Users on v2.45.2.vfs.0.2 (where git restore . overhydrates), they can use git reset --hard as a better representation of this behavior.
2. Users on any Git version can use git restore --ignore-skip-worktree -- <pathspec> when using a smaller-scale pathspec that corresponds to files they care about and probably have hydrated.

I will include these workarounds in the VFS for Git issue.

To get to a real solution here, we would somehow need to turn the ignore_skip_worktree bit into two bits:

a. A bit to say "update index entries that have SKIP_WORKTREE set".
b. A bit to say "update the working directory even if SKIP_WORKTREE is set".

And perhaps this "working directory" version is what was intended when adding the --ignore-skip-worktree option, but it's more involved and has upstream implications to make changes here.

[dscho]

Makes sense. It's sad that we cannot keep the VFS for Git bug fix, but the regression is too bad not to revert. Thank you for diagnosing this so thoroughly and fixing it!