bisect: avoid copying ancestor list for non-merge commits During a bisection, hg needs to compute a list of all ancestors for every candidate commit. This is accomplished via a bottom-up traversal of the set of candidates, during which each revision's ancestor list is populated using the ancestor list of its parent(s). Previously, this involved copying the entire list, which could be very long in if the bisection range was large. To help improve this, we can observe that each candidate commit is visited exactly once, at which point its ancestor list is copied into its children's lists and then dropped. In the case of non-merge commits, a commit's ancestor list consists exactly of its parent's list plus itself. This means that we can trivially reuse the parent's existing list for one of its non-merge children, which avoids copying entirely if that commit is the parent's only child. This makes bisections over linear ranges of commits much faster. During some informal testing in the large publicly-available `mozilla-central` repository, this noticeably sped up bisections over large ranges of history: Setup: $ cd mozilla-central $ hg bisect --reset $ hg bisect --good 0 $ hg log -r tip -T '{rev}\n' 628417 Test: $ time hg bisect --bad tip --noupdate Before: real 3m35.927s user 3m35.553s sys 0m0.319s After: real 1m41.142s user 1m40.810s sys 0m0.285s

rhg-status: add support for narrow clones

rust: add support for hints in error messages This will be used by the narrow support code in the next commit.

rust: add Debug constraint to Matcher trait This makes sure we can easily debug which Matcher we're looking at when using trait objects, and is just generally useful. Effort to make the debugging output nicer has been kept to a minimum, please feel free to improve.

rhg: add sparse support

rhg: add debugrhgsparse command to help figure out bugs in rhg

rhg-status: extract a function for printing pattern file warnings This will be reused for the warnings produced by the sparse file parsing functions.

rust-filepatterns: allow overriding default syntax This will be used when parsing pattern files other than .hgignore like the sparse spec.

rhg: don't fallback if `strip` or `rebase` are activated Neither of these extensions do anything other than add commands, so ignoring them opens up more of the test suite to rhg.

rhg: fallback in `debugdata` if repo has `narrow` Narrow uses ellipsis nodes and debugdata does not understand them yet.