walk: refactor walk plan - never is gone - reorder tests more cleanly - rename nomatches to exact for clearer semantics

match: remove match.never Only one user, can be translated to match.exact()

walk: use match.bad callback for filetype messages

match: ignore return of match.bad All users returned false, return can now be dropped

add: use match.bad callback more effectively

match: document bad callback semantics

walk: simplify logic for badfn clause - matchfn redundant - call badfn always rather than fwarn - use for/else rather than keep var

walk: we always have a badfn

walk: simplify check for missing file

run-tests: redefine --with-hg so it takes the 'hg' script to run. - in parseargs(), check that --with-hg value is valid - add handy --local option for "--with-hg=../hg" - ensure that we always set PATH and PYTHONPATH (not just when doing a temporary install) - override any existing PYTHONPATH, so test success does not depend on whatever happens to be in the caller's environment - give tests a little more control by exporting $PYTHON to the environment; needed by test-convert and test-mergetool when they run hg with a stripped-down $PATH Also, add a big comment explaining all the corner cases to test for the next person who tries to modify this script.