githelp: fail gracefully in a couple cases where arguments are missing I didn't bother adding tests because the other commands that already handled missing arguments don't test these edge cases. I didn't read over all of the code, rather I scanned for `args` not being checked before indexing.

revlog: isgooddeltainfo takes the whole revinfo object Future changes will need other information about te revision.

revlog: make variable name 'd' more explicit in _isgooddeltainfo d -> deltainfo

debugdeltachain: r.start and r.length can be retrieved outside the loop