delta-find: split the "sparse" part of `_pre_filter_rev` in a method
Since `_pre_filter_rev` contains logic from various sources of constraint, we
start splitting is in subfunction to clarify and document the grouping.
changeset: {node|short}
user: {author|person}
date: {date|age}
summary: {desc|firstline|nonempty}
{branches%branchname}{tags%tagname}{bookmarks%bookmarkname}
node: ({col}, {row}) (color {color})
{edges%graphedge}