amend: add option to update to the current user
This is also from the evolve extension's version of amend. A side effect of
this refactoring is for uncommit to support `rewrite.update-timestamp`.
Differential Revision: https://phab.mercurial-scm.org/D6853
== New Features ==
* The amend extension supports the `--currentuser` 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 ==