hgweb: filter graphmod.colored() output before iterating over it
Consumers in this function use output of graphmod.colored(), but only want
items with type == CHANGESET, so let's filter it early.
This is primarily just a refactoring, but it also fixes a potential small bug
with `rows = len(tree)` (this variable is used for "Rows shown" line in
raw-graph) if there are items of other types.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}