repoview: add caching bits Add a caching infrastructure to cache hidden changesets. The cache tries to read the cache lazily and falls back to recomputing if no wlock can be obtain. To validate the cache we store a sha of the obstore content and repo heads in the beginning of the cache which we check every request.

repoview: split _gethiddenblockers Split up _gethiddenblockers into two categories: (1) "static' blockers that solely rely on the contents of obstore and are visible children of hidden changsets. (2) "dynamic" blockers, appearing by having wd parents, bookmarks or tags pointing to hidden changesets. We assume that (1) doesn't change often and can be easily cached with a good invalidation strategy. (2) change often, but barely produce blockers, so we can recompute them if necessary.

repoview: use set for blockers Blockers should be unique but tags and bookmarks could point to the same rev, therefore use a set to ensure that we don't have duplicates.

histedit: preserve initial author on fold (issue4296) When the authorship of the changeset folded in does not match that of the base changeset, we currently use the configured ui.username instead. This is especially surprising when the user is not the author of either of the changesets. In such cases, the resulting authorship (the user's) is clearly incorrect. Even when the user is folding in a patch they authored themselves, it's not clear whether they should take over the authorship. Let's instead keep it simple and always preserve the base changeset's authorship. This is also how "git rebase -i" handles folding/squashing.