gather a summary's commits once instead of seven times

[nepinhum]

summary asked seven questions about one set of commits and with() reselected that set for each of them. On a 200k commit index the seven queries summed to 0.82s, which was the whole command.

Gather the set once into a temp table and read from it. The counterpart role keeps its own scan: it is a different set and that is one scan for one number.

• the table keeps only id, date and time. The object id and subject belong to two commits out of however many the set holds, so those two are looked up when they are asked for.
• repositories is the number of groups top_repositories already computes, so one grouping now answers both. That removed a query rather than adding one.

	200k commits, 50 identities, 4 repositories
before	0.77-0.81 s
gather once	0.43-0.45 s
narrow set, one grouping	0.33-0.34 s

Closes #8.