SCRIPTS: git-show-backports: add -H to use the hash of the commit message

SCRIPTS: git-show-backports: add -H to use the hash of the commit message

Sometimes certain commits don't contain useful tracking information but
we'd still like to be able to report them. Here we implement a hash on
the author's name, e-mail and date, the subject and the body before the
first s-o-b or cherry-picked line. These parts are supposed to be
reasonable invariant across backports and are usable to compute an
invariant hash of a given commit. When we don't find ancestry in a
commit, we try this method (if -H is specified) to compare commits
hashes and we can report a match. The equivalent commit is reported
as "XXXX+?" to indicate that it's an apparent backport but we don't
know how far it goes.