Manifest "group" sections don't handle non-"+"/"-" lines well

[kmod]

I was writing a "group" section and I just wrote out the names of the benchmarks, but that crashed pyperformance with

  File "/home/kmod/pyston3/build/system_env/lib/python3.8/site-packages/pyperformance/_manifest.py", line 365, in _parse_group_section
    yield op, name
UnboundLocalError: local variable 'op' referenced before assignment

It looks like the non-+/- case doesn't set the variable op:

pyperformance/pyperformance/_manifest.py

Line 363 in 94310a3

name = line

My assumption was that these should behave like + lines but I wasn't sure