check-code: allow command substitution with $(command)
Both `command` and $(command) are specified by POSIX. The latter nests
better. I don't see why we shouldn't allow both (or only the latter).
Differential Revision: https://phab.mercurial-scm.org/D6789
== New Features ==
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==