Bisect: add checks at the beginning of "git bisect run".

We may be able to "run" with only one good revision given
and then verify that the result of the first run is bad.
And perhaps also the other way around.

But for now let's check that we have at least one bad and
one good revision before we start to run.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-bisect.sh

@@ -223,6 +223,14 @@ bisect_replay () {
 }
 
 bisect_run () {
+    # Check that we have everything to run correctly.
+    test -d "$GIT_DIR/refs/bisect" || {
+	echo >&2 'You need to start by "git bisect start".'
+	echo >&2 'And then by "git bisect bad" and "git bisect good".'
+	exit 1
+    }
+    bisect_next_check fail
+
     while true
     do
       echo "running $@"