configitems: register the full 'merge-tools' config and sub-options We register the merge-tools config section (which has an arbitrary base config value) and the possible sub-attribute. The sub-attribute has to be registered first or at the same time otherwise the '.*' item would shadow them. Merge tools could include "." in their name so we can't constrain any more than just ".*".

revlog-sparse-read: add a lower-threshold for read block size The option experimental.sparse-read.min-block-size specifies the minimal size of a deltachain span, under which it won't be split by _slicechunk.

revlog: introduce an experimental flag to slice chunks reads when too sparse Delta chains can become quite sparse if there is a lot of unrelated data between relevant pieces. Right now, revlog always reads all the necessary data for the delta chain in one single read. This can lead to a lot of unrelated data to be read (see issue5482 for more details). One can use the `experimental.maxdeltachainspan` option with a large value (or -1) to easily produce a very sparse delta chain. This change introduces the ability to slice the chunks retrieval into multiple reads, skipping large sections of unrelated data. Preliminary testing shows interesting results. For example the peak memory consumption to read a manifest on a large repository is reduced from 600MB to 250MB (200MB without maxdeltachainspan). However, the slicing itself and the multiple reads can have an negative impact on performance. This is why the new feature is hidden behind an experimental flag. Future changesets will add various parameters to control the slicing heuristics. We hope to experiment a wide variety of repositories during 4.4 and hopefully turn the feature on by default in 4.5. As a first try, the algorithm itself is prone to deep changes. However, we wish to define APIs and have a baseline to work on.