tests: more comment cleanup related to obsolescence

test: fix in-test comments related to obsolescence The `ui.prevent-unstable` option never made it into core. It always behaves this way when obsolescence feature is enabled. See changesets c5bd753c5bc6, bacf55bd8f90 and 0f5a0a2073a8 for details.

stream_in: write the remote branchcache for a usage as wide as possible Writing cache for unfiltered repo only is barely useful, Most repo user are now at least use the `hidden` filter. This changeset now assigns the remote cache for a filter as low as possible for a wider reuse as possible.

destroyed: update `unserved` branchcache instead Before this changesets the `destroyed` function updated the branchcache for unfiltered repository. As seen in a previous changeset, Read only repo does not cares about the unfiltered repo. We now update it for `unserved`.

destroyed: drop complex branchcache rebuilt logic The strip code used a trick to lower the cost of branchcache update after a strip. However is less necessary since we have branchcache collaboration. Invalid branchcache are likely to be cheaply rebuilt again a near subset of the repo. Moreover, this trick would need update to be relevant in the now filtered repository world. It currently update the unfiltered branchcache that few people cares about. Make it smarter on that aspect would need complexes update of the calling logic So this mechanism is: - Arguably needed, - Currently irrelevant, - Hard to update and I'm dropping it. We now update the branchcache in all case by courtesy of the read only reader. This changeset have a few expected impact on the testsuite are different cache are updated.

branchmap: update cache of 'unserved' filter on new changesets The `commitctx` and `addchangegroup` methods of repo upgrade branchcache after completion. This behavior aims to keep the branchcache in sync for read only process as hgweb. See ee317dbfb9d0 for details. Since changelog filtering is used, those calls only update the cache for unfiltered repo. One of no interest for typical read only process like hgweb. Note: By chance in basic case, `repo.unfiltered() == repo.filtered('unserved')` This changesets have the "unserved" cache updated instead. I think this is the only cache that matter for hgweb. We could imagine updating all possible branchcaches instead but: - I'm not sure it would have any benefit impact. It may even increase the odd of all cache being invalidated. - This is more complicated change. So I'm going for updating a single cache only which is already better that updating a cache nobody cares about. This changeset have a few expected impact on the testsuite are different cache are updated.

tests: make test-hgweb.t output stable Instability introduced in combination of a4d7fd7ad1f7 and e389a25e7e60.

hgweb: ensure _navseq yield strictly increasing numbers This is not hard and allows to drop hack in the customer code.

hgweb: better names for _navseq arguments The old names were misleading.

hgweb: drop recursivity in _navseq This is totally not needed.