Use only the first line of the commit message on chat service notification

From !23385 (comment 279834501)

Summary

Push commit notification to chat services (Discord, HangoutsChat, Mattermost, MicrosoftTeams, Slack, and UnifyCircuit (as of 12.7)) can be too long when one or more commit message(s) have multiple line in the new commits.

What is the current bug behavior?

Whole commit messages are shown in chat notifications.

What is the expected correct behavior?

Only the first lines of commit message (almost same as commit message title) are shown in chat notifications.

Problems

Due to this problem, we had to have workaround fix on gitlab-foss!32180 (merged) (gitlab-foss#65389 (closed))