git-submodule.sh: clarify the "should we die now" logic

Earlier the decision to stop or continue was made on the $action variable
that was set by inspecting $update_module variable. The former is a
redundant variable and will be removed in another topic.

Decide upon inspecting $update_module if a failure should cascade up to
cause us immediately stop, and use a variable that means just that, to
clarify the logic.

Incidentally this also makes the merge with the other topic slightly
easier and cleaner to understand.

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

git-submodule.sh

@@ -508,16 +508,19 @@ cmd_update()
 				update_module= ;;
 			esac
 
+			must_die_on_failure=
 			case "$update_module" in
 			rebase)
 				command="git rebase"
 				action="rebase"
 				msg="rebased onto"
+				must_die_on_failure=yes
 				;;
 			merge)
 				command="git merge"
 				action="merge"
 				msg="merged in"
+				must_die_on_failure=yes
 				;;
 			*)
 				command="git checkout $subforce -q"
@@ -529,16 +532,12 @@ cmd_update()
 			if (clear_local_git_env; cd "$path" && $command "$sha1")
 			then
 				say "Submodule path '$path': $msg '$sha1'"
-			else
-				case $action in
-				rebase|merge)
+			elif test -n "$must_die_on_failure"
+			then
 				die_with_status 2 "Unable to $action '$sha1' in submodule path '$path'"
-					;;
-				*)
+			else
 				err="${err};Failed to $action in submodule path '$path'"
 				continue
-					;;
-				esac
 			fi
 		fi