gitlint: Add regex to ignore some long lines.

This does not resolve issues for all previous commits where the B1 rule
of long lines in the body would fail, but will allow some obvious common
cases to pass.

Inspired by jorisroovers/gitlint#255, this should cover at least lines
starting with:
- http or https [where URLs can be long]
- Co-authored-by: [where emails/names can be long]

The downside to this rule is that the line numbering is apparently
offset, based on the gitlint documentation.
(https://jorisroovers.com/gitlint/latest/ignoring_commits/)

Author: neiljp

Diff for: .gitlint

@@ -55,3 +55,6 @@ ignore=all
 # Ignore certain rules, you can reference them by their id or by their full name
 # Use 'all' to ignore all rules
 # ignore=T1,body-min-length
+
+[ignore-body-lines]
+regex=^(Co-authored-by:| *https?://\S+$)