Merge branch 'cb/rebase-am-exit-code' into maint

"git rebase" did not exit with failure when format-patch it invoked
failed for whatever reason.

* cb/rebase-am-exit-code:
  rebase: return non-zero error code if format-patch fails

Author: gitster

git-rebase--am.sh

@@ -78,7 +78,7 @@ else
 
 		As a result, git cannot rebase them.
 		EOF
-		return $?
+		return $ret
 	fi
 
 	git am $git_am_opt --rebasing --resolvemsg="$resolvemsg" \