tests: add regression tests for another revrange edge case These tests would've passed before ac0c12123743 and e441657b372b. Inserting them to make sure that continues to be the case.

graphmod: don't try to visit nullrev (issue3772)

log: remove any ancestors of nullrev (issue3772) For the special case, ":null" we remove the implied revision 0 since that wouldn't make any sense here. A test case is added to make sure only nullrev is shown.

help: add documentation for new template functions

changectx: fix the handling of `tip` We can not use `len(repo,changelog)`, it may be a filtered revision. We now use `repo,changelog.tip()` to fetch this information. The `tip` command is also fixed and tested Thanks goes to Idan Kamara for the initial report.

bisect: use changelog for iteration With changelog filtering, we can not use xrange anymore. We have to use the changelog to do the iteration. This way, the changelog excludes filtered revision and we can safely use what we iterate over. Without this changes, bisect crash with a traceback if there is filtered revision in the repo. Tests have been updated.