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.
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