tests/helper-runtests.sh

rebase: provide detailed hint to abort message if working dir is not clean

Detailed hint message is now provided when 'pull --rebase' operation detects
unclean working dir, for example:
  abort: uncommitted changes
  (cannot pull with rebase: please commit or shelve your changes first)

Added tests for uncommitted merge, and for subrepo support verifying that same
hint is also passed to subrepo state check.

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL