.clang-format

help: show "[no-]" only for default-on Flags

As Anton (av6) pointed out, the "[no-]" is confusing for action flags
like `hg bookmark --delete`. We could come up with a way of indicating
which flags are action flags (e.g. use None for the default value
instead of False). However, it's probably also unlikely that users
will want to negate even non-action flags like --hidden.

One of the more common flags where the "[no-]" prefix would be useful
is `hg evolve --update`. The reason it's helpful there is that it
defaults to on. So I think we can simply include "[no-]" only for
flags that are on by default (and thus require the user to add the
"[no-]" for the option to have any effect).

Note that there are use cases for negating flags that already off by
default. For example, you may have an alias for `hg log -G --hidden -T
foo` and now want to pass "--no-hidden" to that alias. However, I
think that users who want that are likely to be advanced enough that
they've already learnt about the "no-" prefix by seeing it somewhere
else.

Differential Revision: https://phab.mercurial-scm.org/D5454

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