impact of is_ignored on mutabletree.smart_add should be checked

[Parth Malwankar]

Based on John's comment on https://code.launchpad.net/~parthm/bzr/300062-2.2-bad-pattern-error-part-2/+merge/29949
the impact of is_ignored on mutabletree.smart_add should be checked as there may be room for improvement.

Quote:
> I may be over-concerned, but I do believe that 'is_ignored()' is a fair
> amount of overhead in the 'initial add of 30k files', and I'd like us to
> at least be aware of it. I do wonder about changing the internals so
> that instead of using '?:.*/)?(?!.*/)) to filter out everything but the
> basename for some patterns, if we wouldn't be better off knowing
> that this is a basename-only pattern and just supply that to the match.