fileset: add "subrepo" fileset symbol
This new fileset symbol returns a list of subrepos whose paths match a given
pattern. If the argument has no pattern type set, an exact
match is performed.
If no argument is passed, return a list of all subrepos.
Ancestors of 5
4 2 0
Ancestors of 6 and 5
3 4 2 1 0
Ancestors of 5 and 4
4 2 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8