sparse-read: skip gaps too small to be worth splitting Splitting at too small gaps might not be worthwhile. With this changeset, we stop considering splitting on too small gaps. The threshold is configurable. We arbitrarily pick 256K as a default value because it seems "okay". Further testing on various repositories and setups will be needed to tune it. The option name is 'experimental.sparse-read.min-gap-size`, and replaces `experimental.sparse-read.min-block-size` which is not used any more.

sparse-read: move from a recursive-based approach to a heap-based one The previous recursive approach was trying to optimise each read slice to have a good density. It had the tendency to over-optimize smaller slices while leaving larger hole in others. The new approach focuses on improving the combined density of all the reads, instead of the individual slices. It slices at the largest gaps first, as they reduce the total amount of read data the most efficiently. Another benefit of this approach is that we iterate over the delta chain only once, reducing the overhead of slicing long delta chains. On the repository we use for tests, the new approach shows similar or faster performance than the current default linear full read. The repository contains about 450,000 revisions with many concurrent topological branches. Tests have been run on two versions of the repository: one built with the current delta constraint, and the other with an unlimited delta span (using 'experimental.maxdeltachainspan=0') Below are timings for building 1% of all the revision in the manifest log using 'hg perfrevlogrevisions -m'. Times are given in seconds. They include the new couple of follow-up changeset in this series. delta-span standard unlimited linear-read 922s 632s sparse-read 814s 566s

subrepo: implement 'unshare' for Mercurial subrepos I think there's a slight hole here in that a subrepo could be shared, removed from .hgsub, and then it's not part of context.substate (so not iterated over). But the push command has the same hole IIRC, and I think removing a subrepo is an edge case. The import hack is a copy/paste of subrepo.subrepo().