relnotes: we now require `sh` to support $(command) syntax to run test suite
For example, Solaris before version 11 had /bin/sh pointing to the old
Bourne Shell (which doesn't support $(command) syntax).
Differential Revision: https://phab.mercurial-scm.org/D6833
== New Features ==
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
* A shell that supports `$(command)`` syntax for command substitution is now
required for running the test suite. The test runner normally uses
`sh`, so if that is a shell that doesn't support `$(command)` syntax,
you can override it by setting `$HGTEST_SHELL` or by passing it to
`run-tests.py --shell <shell>`.
== Internal API Changes ==