git-submodule summary: fix that some "wc" flavors produce leading spaces

We print the number of commits in parentheses, but without this change
we would get an oddly looking line like this:

    * sm1 4c8d358...41fbea9 (      4):
Signed-off-by: NJohannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -448,7 +448,7 @@ cmd_summary() {
 			GIT_DIR="$name/.git" \
 			git log --pretty=oneline --first-parent $range | wc -l
 			)
-			total_commits=" ($total_commits)"
+			total_commits=" ($(($total_commits + 0)))"
 			;;
 		esac