bisect: further style nitpicks

Fix a few remaining lines that indented with spaces.

Also simplify the logic of checking out the original branch and reporting
error during "bisect reset".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -364,14 +364,12 @@ bisect_reset() {
 	*)
 		usage ;;
 	esac
-	if ! test -f "$GIT_DIR/BISECT_HEAD"
-	then
-		if ! git checkout "$branch" --
+
+	if ! test -f "$GIT_DIR/BISECT_HEAD" && ! git checkout "$branch" --
 	then
 		die "$(eval_gettext "Could not check out original HEAD '\$branch'.
 Try 'git bisect reset <commit>'.")"
 	fi
-	fi
 	bisect_clean_state
 }