doc.docbook.mk: Fix LATESTREVISION determination, causes build failure on non-English locales
Fixes, e.g., manual articles/books build (i.e., typing make in the right
directory).
There were two problems:
- Git or SVN information retrieval depends on the current locale, and doesn't
work with non-English ones.
- Fallback for LATESTREVISION most of the time (sometimes works, probable
race) itself fails because PWD is set to document's directory + '/obj', but
sources are in the document's directory.
Problem 2 is the most annoying because documentation builds simply fails
(xsltproc becomes unable to resolve 'latestrevision.*' variables in, e.g.,
articles). But point 2's failure appears only because of point 1's failure,
which doesn't happen on a machine with English or C locale.
PR: 246936