index: handle index[-1] as nullid more explicitly I find it more intuitive to check if "pos == -1" than to first add the index length (which includes one extra item for the nullid) and compare that to "length - 1". However, because test-parseindex2.py compares the whole index (up to len(index)-1), we need to also preserve that other check for a little while more. I'll remove it soon. Differential Revision: https://phab.mercurial-scm.org/D4018

index: store nullrev as -1 in nodetree Nothing important, it just seems more natural to not map nullrev to INT_MAX. We just need to change the revision encoding a little to make space for the -1. Differential Revision: https://phab.mercurial-scm.org/D4005

index: create function for deleting node from nodetree A node is marked deleted by nt_insert(self, node, -1), but "-1" could mean so many things (notably nullrev), so let's hide it in a function. The function could possibly also remove nodes from the tree in the future (it currently only marks them deleted). Differential Revision: https://phab.mercurial-scm.org/D4004

revlog: remove some knowledge of sentinel nullid in index I think the "-2" to mean "last position in index, not counting the null revision at the end" is an implementation detail of the index that we should avoid spreading knowledge of. I hope we can even remove support for index[-2]. Differential Revision: https://phab.mercurial-scm.org/D4016

scmutil: make shortest() respect disambiguation revset The previous patch would let you use a shorter prefix if the prefix is unique within a configured revset. However, that's not very useful if there's no simple way of knowing what that shorter prefix is. This patch adapts the shortest() template function to use the shorter prefixes for nodes in the configured revset. This is currently extremely slow, because it calculates the revset for each call to shortest(). To make this faster, the next patch will start caching the revset instance. Ideally we'd cache a prefix tree instance instead. Differential Revision: https://phab.mercurial-scm.org/D4038

lookup: add option to disambiguate prefix within revset When resolving a nodeid prefix that is not unique within the repo and the user has configured a revset that they want to disambiguate within, we now try to look up within that revset before we fail. If there is a unique match within the revset, we use that. This is of course most effective at allowing a short prefix if the revset contains few nodes. For most of our internal users at Google, "not public()" is sufficiently small that a hex digit or two is enough. The implementation is currently pretty slow, but good enough for small revsets (which is the expected use case). The scan in the revset is linear. We may want to use a prefix tree if we want to allow users to use a larger revset. Credit for the idea goes to Kyle Lippincott. Differential Revision: https://phab.mercurial-scm.org/D4037

revlog: use specialized exception for ambiguous prefix lookup It's useful to be able to catch a specific exception for this case. We'll use it soon. Differential Revision: https://phab.mercurial-scm.org/D4036