tests/helper-runtests.sh

transaction: avoid ambiguity of file stat at restoring from backup

In some cases below, copying from backup is used to restore original
contents of a file, which is backuped via addfilegenerator(). If
copying keeps ctime, mtime and size of a file, restoring is
overlooked, and old contents cached before restoring isn't invalidated
as expected.

  - failure of transaction (from '.hg/journal.backup.*')
  - rollback of previous transaction (from '.hg/undo.backup.*')

To avoid ambiguity of file stat at restoring, this patch invokes
util.copyfile() with checkambig=True.

This patch is a part of "Exact Cache Validation Plan":

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL