+arg="$1"
+if rev=$(git rev-parse --verify "$arg^0" 2>/dev/null)
+then
+ [ -z "$rev" ] && die "unknown flag $arg"
+ new_name="$arg"
+ if git show-ref --verify --quiet -- "refs/heads/$arg"
+ then
+ rev=$(git rev-parse --verify "refs/heads/$arg^0")
+ branch="$arg"
+ fi
+ new="$rev"
+ shift
+elif rev=$(git rev-parse --verify "$arg^{tree}" 2>/dev/null)
+then
+ # checking out selected paths from a tree-ish.
+ new="$rev"
+ new_name="$arg^{tree}"
+ shift
+fi
+[ "$1" = "--" ] && shift
+