amend: enable support for using the secret phase
This comes from the evolve extension's version of amend. The logic was already
in place, and appears to be the last of the trivial things that can be enabled.
Differential Revision: https://phab.mercurial-scm.org/D6856
== New Features ==
* The amend extension supports the `--currentuser` argument.
* The amend extension supports the `--close-branch` argument.
* The amend extension supports the `--secret` argument.
* The uncommit extension supports the `rewrite.update-timestamp` config option.
== 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 ==