checkpatch.pl: add check for Change-Id

checkpatch.pl: add check for Change-Id

A commit hook for the Gerrit code review server [1] inserts change
identifiers so Gerrit can track patches through multiple revisions.
These identifiers are noise in the context of the upstream kernel.
(Many Gerrit servers are private.  Even given a public instance, given
only a Change-Id, one must guess which server a change was tracked on.
Patches submitted to the Linux kernel mailing lists should be able to
stand on their own.  If it's truly useful to reference code review on a
Gerrit server, a URL is a much clearer way to do so.) Thus, issue an
error when a Change-Id line is encountered before the Signed-off-by.

1. https://gerrit.googlesource.com/gerrit/+/master/gerrit-server/src/main/resources/com/google/gerrit/server/tools/root/hooks/commit-msg

Signed-off-by: Christopher Covington <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>