see also issue 1146

Although I don't know much about Python I added -n/--dry-run for the "merge"
command. It at least works for me.

issue558 (qpush should have a --dry-run option) and
issue581 (there's no --noexec option for any mercurial options)
discuss the same topic. Setting superceder for them and moving nosy to this issue.

Vadim added -n/--dry-run for all the working dir related operations.

pull -n is problematic because there is currently no way of getting a list of
changes from the remote repository without getting the full changegroup for all
those changes first.  So, the most likely usage scenario would involve pulling
all the changes twice.  First to see what would be pulled, than again to
actually do the pull.

Of course, implementing incoming for remote repositories has the same problem.

One implementation idea is to implement a message that would cause only the
changelog entries to be pulled and nothing else.  These could then be printed
and thrown away.  This still wastes some bandwidth, but not nearly as much as
the new manifest revisions and file revisions would only be sent if -n wasn't given.

Since pull, and push are currently kind of quiet about what they're doing,
they'd also need an option to be noisier so the needed information could be
retrieved.

Add an option to several commands that modify the working directories or
repository to prevent them from actually making that change.  Instead, they will
print out the change they would've made.

This would allow incoming and outgoing to be eliminated in favor of push -n and
pull -n.  Other commands could possibly be eliminated as well.  update would be
modified to allow it to print out a list of the working directory files it would
change instead of actually changing them.