rebase: fix code comment to refer to right issue (4504, not 4505)
The comment was introduced in 8a544fb645bb (rebase: ensure rebase
revision remains visible (issue4504), 2015-01-27), which mentions the
right issue in the description.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL