tests/test-gendoc.t

setup: make the error "Unable to find a working hg binary" more informative

The error message now shows the attempted hg commands and their stderr, to
make it easier to investigate why things are not working.

Here's an example output

/!\
/!\ Unable to find a working hg binary
/!\ Version cannot be extracted from the repository
/!\ Re-run the setup once a first version is built
/!\ Attempts:
/!\   attempt #0:
/!\     cmd:         ['hg-missing', 'log', '-r.', '-Ttest']
/!\     exception:   [Errno 2] No such file or directory: 'hg-missing': 'hg-missing'
/!\   attempt #1:
/!\     cmd:         ['/usr/bin/python3', 'hg', 'log', '-r.', '-Ttest']
/!\     return code: 255
/!\     std output:
/!\     std error:
*** failed to import extension "topic": No module named 'topic'
*** failed to import extension "evolve": No module named 'evolve'
abort: accessing `dirstate-v2` repository without associated fast implementation.
(check `hg help config.format.use-dirstate-v2` for details)
/!\
/!\ Could not determine the Mercurial version
/!\ You need to build a local version first
/!\ Run `make local` and try again
/!\

#require docutils
#require gettext

Test document extraction

  $ HGENCODING=UTF-8
  $ export HGENCODING
  $ { echo C; ls "$TESTDIR/../i18n"/*.po | sort; } | while read PO; do
  >     LOCALE=`basename "$PO" .po`
  >     echo "% extracting documentation from $LOCALE"
  >     LANGUAGE=$LOCALE "$PYTHON" "$TESTDIR/../doc/gendoc.py" >> gendoc-$LOCALE.txt 2> /dev/null || exit
  > 
  >     if [ $LOCALE != C ]; then
  >         if [ ! -f $TESTDIR/test-gendoc-$LOCALE.t ]; then
  >             echo missing test-gendoc-$LOCALE.t
  >         fi
  >         cmp -s gendoc-C.txt gendoc-$LOCALE.txt && echo "** NOTHING TRANSLATED ($LOCALE) **"
  >     fi
  > done; true
  % extracting documentation from C
  % extracting documentation from da
  % extracting documentation from de
  % extracting documentation from el
  % extracting documentation from fr
  % extracting documentation from it
  % extracting documentation from ja
  % extracting documentation from pt_BR
  % extracting documentation from ro
  % extracting documentation from ru
  % extracting documentation from sv
  % extracting documentation from zh_CN
  % extracting documentation from zh_TW