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.
To run the tests, do:
cd tests/
python run-tests.py
See http://mercurial.selenic.com/wiki/WritingTests for
more information on writing tests.