Add "git-patch-id" program to generate patch ID's.

Add "git-patch-id" program to generate patch ID's.

A "patch ID" is nothing but a SHA1 of the diff associated with a patch,
with whitespace and line numbers ignored.  As such, it's "reasonably
stable", but at the same time also reasonably unique, ie two patches
that have the same "patch ID" are almost guaranteed to be the same
thing.

IOW, you can use this thing to look for likely duplicate commits.