Fix the COVERAGE_IS_PARTIAL Status Text on the Carry Paths in pr_review.py #3424

The status=COVERAGE_IS_PARTIAL line in report_verdict says the review covering the head states it read fewer files than the pull request changed. On the carry paths (an earlier partial beside a head stating nothing, or a head reading unknown) the bracketed reason says the opposite, so the line contradicts itself. Constructed case: earlier round reviewed=1 changed=3, head round reviewed=4 changed=3. Found by the local strict review of #3423.