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The `signed-off-by` rule does not correctly detect the `Signed-off-by:`
trailer if it's not the last trailer. Therefore, this rule has been
disabled until we can resolve this in the commitlint upstream.

Change-Id: I50ea29067528f3c1c25beeea5eb25134b25b2af2
Signed-off-by: Chris Kay <chris.kay@arm.com>

This change adds a configuration for commitlint - a tool designed to
enforce a particular commit message style - and run it as part of Git's
commit-msg hook. This validates commits immediately after the editor has
been exited, and the configuration is derived from the configuration we
provide to Commitizen.

While the configuration provided suggests a maximum header and body
length, neither of these are hard errors. This is to accommodate the
occasional commit where it may be difficult or impossible to comply
with the length requirements (for example, with a particularly long
scope, or a long URL in the message body).

Change-Id: Ib5e90472fd1f1da9c2bff47703c9682232ee5679
Signed-off-by: Chris Kay <chris.kay@arm.com>