resolve: support confirm config option with --unmark flag
Now, commands.resolve.confirm also respect --unmark option; and
confirm to unresolve all resolved files.
It will confirm only when no files pats are passed (same as --mark),
because when no pats are passed the default is to mark resolved files
as unresolved.
And if user has passed file pats then I think there is no need to confirm
for that.
Differential Revision: https://phab.mercurial-scm.org/D4102
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