lint: avoid false positives with URLs at start of line in body

When the message body's last line starts with a URL, lint thought this
was a footer line. This change blacklists URLs using the same regexp
as the Gerrit commit-msg hook, by matching ^[a-ZA-Z0-9-]:// (i.e. a
footer field name directly followed by two slashes)

Change-Id: I061bee679e7ec807fa0d7c233fbbf432faa268c0