bisect reset: Leave the tree in usable state if git-checkout failed

I had local modifications in the tree and doing bisect reset required me to
manually edit .git/HEAD.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-bisect.sh

@@ -179,11 +179,12 @@ bisect_reset() {
         *)
 	    usage ;;
 	esac
-	git checkout "$branch" &&
-	rm -fr "$GIT_DIR/refs/bisect"
-	rm -f "$GIT_DIR/refs/heads/bisect" "$GIT_DIR/head-name"
-	rm -f "$GIT_DIR/BISECT_LOG"
-	rm -f "$GIT_DIR/BISECT_NAMES"
+	if git checkout "$branch"; then
+		rm -fr "$GIT_DIR/refs/bisect"
+		rm -f "$GIT_DIR/refs/heads/bisect" "$GIT_DIR/head-name"
+		rm -f "$GIT_DIR/BISECT_LOG"
+		rm -f "$GIT_DIR/BISECT_NAMES"
+	fi
 }
 
 bisect_replay () {