Only provide the commit summary for Discord webhook push events (#32432)

Only provide the commit summary for Discord webhook push events (#32432)

Resolves #32371.

#31970 should have just showed the commit summary, but
`strings.SplitN()` was misused such that we did not perform any
splitting at all and just used the message. This was not caught in the
unit test made in that PR since the test commit summary was > 50 (which
truncated away the commit description).

This snapshot resolves this and adds another unit test to ensure that we
only show the commit summary.