dirstate._normalize: don't construct dirfoldmap if not necessary
Constructing the dirfoldmap is expensive, so if there's a hit in the
filefoldmap, don't construct the directory foldmap.
This helps with cases like 'hg add foo' where foo is already tracked: for a
large repository, the operation goes from 1.5 seconds to 1.2 (which is still
way too much, but that's a matter for another day.)
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