Allow the hints file to apply a correct timestamp to
the Guidebooks prior to use.
Detect the NH_DATESUB in the Guidebook.mn or Guidebook.tex
files and replace the datestamping line that follows.
If git is available, it determines the hash of the last commit
applied to doc/Guidebook.mn, and then determines the date of
that commit. The interim Guidebook.dated.mn (or
Guidebook.dated.tex) gets the datestamp applied.
If git isn't available or doesn't correctly provide the hash
for doc/Guidebook.mn, it should just continue to use whatever
hard-coded date in the Guidebook.mn source file (it uses it
instead of the interim file).
