sparse-revlog: protect C code against delta chain including nullrev For unclear reasons, some repositories include nullrev (-1). Re-computing delta for such repo remove nullrev from all chain, so some older versions have been creating them. This currently raise an IndexError with the new C code doing chain slicing as it expect all item to be positive. Both python and C code for reading delta chain preserve nullrev, and the Python code for chain slicing handle the case fine. So we take the safe route and make the new C code works fine in that case.

sparse-revlog: handle nullrev in index_get_length The more generic index_get method handle nullrev fine, we apply the same logic here.

sparse-revlog: handle nullrev in index_get_start The more generic index_get method handle nullrev fine, we apply the same logic here.

revlog: introduce a constant for nullrev in `revlog.c` The value is important enough to be explicitly tracked.

rust: translation of missingancestors This is as direct as possible a translation of the ancestor.missingancestors Python class in pure Rust. The goal for this changeset is to make it easy to compare with the Python version. We also add to Python tests the cases that helped us develop and debug this implementation. Some possible optimizations are marked along the way as TODO comments Differential Revision: https://phab.mercurial-scm.org/D5416

contrib: provide a small script that draw performance plot We have been using this script to look into the result of various runs of the `hg perfrevlogwrite` command. It seems useful enough to be shared more widely.

delta: filter nullrev out first When picking a potential candidate, we filter them on various criteria. The "different from nullrev" criteria is very fast to compute and we should process it first.

perf: report more of the higher range in perfrevlogwrite Since the delta chain length is limited to 1000 revisions, we get a new snapshot about every 1000 revisions. If we assume that the snapshot will be most of the slowest revision, the current display (99% and max) are not very precise in their area. We now include more information about this space in the default report.

help: present boolean arguments as "--[no-]foo" This should make it much more discoverable (we document it in `hg help flags`, but most users don't think to look there). Note that flags that default to None (and not False) will not get this new presentation. We can change the defaults to False later for flags where it makes sense (probably almost all boolean flags). Differential Revision: https://phab.mercurial-scm.org/D5432

help: show "default: off" for boolean flags that default to off Differential Revision: https://phab.mercurial-scm.org/D5431