Extend "git reset" to take a reset point

This was triggered by a query by Sam Ravnborg, and extends "git reset" to reset the index and the .git/HEAD pointer to an arbitrarily named point.

For example

	git reset HEAD^

will just reset the current HEAD to its own parent - leaving the working directory untouched, but effectively un-doing the top-most commit. You might want to do this if you realize after you committed that you made a mistake that you want to fix up: reset your HEAD back to its previous state, fix up the working directory and re-do the commit.

If you want to totally un-do the commit (and reset your working directory to that point too), you'd first use "git reset HEAD^" to reset to the parent, and then do a "git checkout -f" to reset the working directory state to that point in time too.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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This is potentially a dangerous command, so maybe we should make it ask for confirmation first?

On the other hand, it definitely is convenient: I often end up doing this by hand, and clearly other people have hit the "oops, I want to undo and then re-do those last five commits, I was just a bit too drunk"

The old "git reset" only reset the index to the current HEAD, which is really only useful if you've tried to do a "merge" that failed and that you're giving up on. This one is more useful, but also potentially more dangerous - doing a

	git reset v0.99.3
	git checkout -f

will basically revert a tree to some old state, and if you didn't save the old point, you may not be able to get back to it (git-fsck-cache will help you, but..)

Not hugely tested, btw. That strange extra "git-rev-parse" is _meant_ to make sure that if you reset to a tag, it will always extract the commit ID from that tag and not reset the HEAD to a tag object.