tests: show failure to `hg add -I` a dir->symlink transition

setup: try a non-pure version of the local Mercurial if the pure fails Things like `zstd` can make the pure version fails.

setup: make debug simpler by adding a `__repr__` to `hgcommand` This help when trying to debug this logic.

rhg: support rhg status --rev --rev

rust: add a utility function to merge ordered fallible iterators Adding a function merge_join_results_by, a version of itertools::merge_join_by that works on "fallible" iterators (iterators that can produce errors)

rhg: refactor hg status, make the display code usable for non-dirstate status

debugformat: speedup the "plain-cl-delta" check Checking every single revision is too slow, let's use a sampling approach.

templatekw: fix inconsistency of diffstat with diff.merge Previously, `-T'{diffstat}'` was giving stats from the diff against p1, regardless of whether `--config diff.merge=yes` is set. This was inconsistent with `log --patch` which is aware of that option.

util: move diff_parent from logcmdutil to diffutil This function will be used outside of the log command (in templatekw, used by hgweb, for which logcmdutil is not available). Let's move this function together with the rest of the diff-related utils instead.

logcmdutil: return structured diffstat data for json