+
+baserev=$(git rev-parse --verify --quiet "$base"^0)
+if test -z "$baserev"
+then
+ die "fatal: Not a valid revision: $base"
+fi
+
+headrev=$(git rev-parse --verify --quiet "$head"^0)
+if test -z "$headrev"
+then
+ die "fatal: Not a valid revision: $head"
+fi