tests/failfilemerge.py

rust: less set lookups in AncestorsIterator

This uses the boolean return of `HashSet::insert()` to factor
pairs of contains()/insert() into a single insert()

On the mozilla-central repository (450k changesets), I get about a bit more
than 10% better medians in perfancestors (taking the mean of three runs)

Best run for parent changeset:

! wall 0.106474 comb 0.110000 user 0.110000 sys 0.000000 (median of 93)

Best run for this changeset:

! wall 0.093191 comb 0.090000 user 0.090000 sys 0.000000 (median of 100)

Differential Revision: https://phab.mercurial-scm.org/D5942

# extension to emulate interrupting filemerge._filemerge

from __future__ import absolute_import

from mercurial import (
    error,
    extensions,
    filemerge,
)

def failfilemerge(filemergefn,
                  premerge, repo, wctx, mynode, orig, fcd, fco, fca,
                  labels=None):
    raise error.Abort("^C")
    return filemergefn(premerge, repo, mynode, orig, fcd, fco, fca, labels)

def extsetup(ui):
    extensions.wrapfunction(filemerge, '_filemerge',
                            failfilemerge)