[Fab-user] Completed suggestion: skipping executed commands (#7)

[Niklas Lindström]

[These changes are present in my fork of Fabric at
<http://github.com/niklasl/fabric/tree/master>]


== 7. Skipping executed commands ==

Added the `invoke` operator for prevention of multiple calls of commands.

This is a somewhat invasive change, but I hope I've addressed it
better now. Here's how it works now:

My initial version only invokes a command once (unless you call it
manually in code) at all times. Christian had a concern that this may
play ill with at least the `set` command. I thus realized that it
works very bad with commands that take arguments altogether.

Therefore, now anything supplied on the command line is executed even
if repeated. But *all* `_execute_command` invokations, including
these, are remembered, *including* their arguments (see `_args_hash`).
This is currently only used via `invoke`, which will not invoke the
same command+args twice.

(The only, in my opinion minor, "danger" is if `prompt` is used to
possibly change a similar invocation after the command call. Since
skips are also printed, even then the user will be notified of the
skip.)

Note: I won't object much if you find this too complex. I can then
perhaps revert to a `call_once` (and associated `depends`) without
support for arguments at all.. (Although I think it would be nice if
it cooperated with the normal _execute_command.)


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Best regards,
Niklas