git-read-tree: add "--reset" flag

git-read-tree: add "--reset" flag

This is the same as "-m", but it will silently ignore any unmerged
entries, which makes it useful for efficiently forcing a new position
regardless of the state of the current index file.

IOW, to reset to a previous HEAD (in case you have had a failed
merge, for example), you'd just do

	git-read-tree -u --reset HEAD

which will also update your working tree to the right state.

NOTE! The "update" will not remove files that may have been added by the
merge.  Yet.