templater: add subsetparents(rev, revset) function Naming suggestions are welcome. And this could be flagged as an (ADVANCED) function since the primary use case is to draw a graph. This provides all data needed for drawing revisions graph filtered by revset, and allows us to implement a GUI graph viewer in some languages better than Python. A frontend grapher will be quite similar to our graphmod since subsetparents() just returns parent-child relations in the filtered sub graph. Frontend example: https://hg.sr.ht/~yuja/hgv/browse/default/core/hgchangesetgrapher.cpp However, the resulting graph will be simpler than the one "hg log -G" would generate because redundant edges are eliminated. This should be the same graph rendering strategy as TortoiseHg. This function could be implemented as a revset predicate, but that would mean the scanning state couldn't be cached and thus slow. Test cases are split to new file since test-template-functions.t is quite big and we'll need a new branchy repository anyway.