rebase: move local variables 'date' and 'extrafns' to the RR class
This commit moves the following variables, local to the rebase function to be
fields of the rebaseruntime:
-date
-extrafns
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL