aspec: include doc build in ASpec again

Isabelle2020 doesn't allow sharing session directories between the document
session and non-document session. Instead of duplicating things, this commit
pulls the document build back into the ASpec session, but changes the build
such that the git revision is read directly from LaTeX, removing the
superfluous re-build for every git revision change (even when no relevant spec
file changed).

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

Import release snapshot.