localrepo: also fastpath `nullrev` in __getitem__ As explained earlier, nullrev will exist in all repository, we do not need any special checking. Differential Revision: https://phab.mercurial-scm.org/D7480

repoview: add a test to track operation not supposed to trigger filtering This test will be useful to confirm we removed filtering trigger and to prevent it to come back without us noticing. Differential Revision: https://phab.mercurial-scm.org/D7479

repoview: display stack trace along side the debug message When a filter computation is triggered, If --traceback is provided, we will display a traceback in addition to the debug message. Differential Revision: https://phab.mercurial-scm.org/D7478

util: add an optional `prefix` argument to debugstacktrace This is useful when using it in a specific context. Differential Revision: https://phab.mercurial-scm.org/D7477

repoview: add a 'devel.debug.repo-filter' option If set, there will be debug message when a filter computation is triggered. This is going to be useful to remove various filtering trigger and to test they do not get reintroduced. Differential Revision: https://phab.mercurial-scm.org/D7476

localrepo: extract handling of some special value in __getitem__ The value "tip" should always be accessible, otherwise there is some problematic bug in the lower level. So we can access this outside of the general try/catch. If it fails some wider and actual big is in play. Differential Revision: https://phab.mercurial-scm.org/D7475

localrepo: extract handling of some special value in __getitem__ The value "null" will always be present in a repository. So this lookup should always succeed and do not need to be in the general try/catch. Differential Revision: https://phab.mercurial-scm.org/D7474