complain about empty lines between footers

they are completely unnecessary and make things look messy.

the algorithm considers cherry-pick lines as footers, following
convention. it even supports multi-line cherry-picks, such as those i
sometimes assemble when squashing multiple commits.

this change will cause a lot of complaints, because almost everybody
appears to be using a broken commit-msg hook. the one coming from our
gerrit is ok, as is the copy in qtrepotools.

Change-Id: I845ba16a70960beb2ce042b2eaddf22b17edcc25
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
complain about gerrit ids in commit messages outside Change-Id footer

people use them to refer to related commits. they should use stable
sha1s of merged commits instead.

Change-Id: Ic58aa875aa978dc76948521266741258fd8c94aa
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
complain about urls pointing to gerrit reviews and jira tasks

urls to reviews should be replaced by sha1s of merged commits.
urls to jira tasks should be replaced by task-number footers.

the rationale is keeping the history relocatable, and somewhat
independent from the infrastructure's software.
also, urls are pretty noisy.

Change-Id: I4b2c9ded8cbc02ca96daa4a2ff92c98c0a4b05f9
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
complain about multiple [ChangeLog] misuses

Change-Id: I223eaf32782b2c0d2de3a815e0968706bfff1bdb
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>