Embed git revision information in documents

Infrastructure to add git describe output and HEAD commit datestamp
via \commitdesc and \headdate defined in a .ver file, which is about
to have been used by SAMv1.tex.

Also ignore and clean .out, produced by the hyperref package.
Add repository URL and version information

(We've lied about the author date on this commit so as to leave the
date as the last substantive textual change to SAMv1.tex.)