Allow also completion for whitespace prefixed commands.

Allow also completion for whitespace prefixed commands.

We allow to put whitespace or additional ':' before ex commands to avoid
recoding of the commands in the history. But in the completion only the
first ':' was skipped to get the command to apply completion for.
So ': open foo<Tab>' did not start completion, because ' open' is none
known command.
So skip all ':' and whitespace after the first ':' to set the pointer to
the beginning of the command.