git2graph: add back node output support, with simpler/saner semantics

Rationale: generating the nodes file from the edges file is not reasonable in
terms of processing time. For linux.git alone, tr + sort -u can take up to 1
hour, depending on the sort setup. On the other hand outputing (unsorted, but
unique) nodes via git2graph adds near-zero overhead w.r.t. outputing edges.

The sane semantics for nodes/edges selection is to completely separate
filtering. The user is expected to filter nodes *and* edges on the command
line (if desired), and neither trickles to the other. So it is possible to,
say, emit "rev:rev" edges and "dir,cnt"; it is up to the user to select a
reasonable semantics.

It is also now possible to filter *out* all nodes/edges, passing empty strings
as filters. That might be needed when one really wants all and only nodes
corresponding to selected edges; in that case nodes output should be
suppressed, and tr+sort used separately. Note that doing so is not always
desirable, as it excludes singleton nodes, not connected to anything at
all (which do exist!).

This commit partially reverts d2ff3227a240d0a4de043661874c1959cc0b462c