localrepository.status: only acquire wlock if actually needed. This speeds up the common case of not needing to update the dirstate, and avoids the need to reload and parse the dirstate "just in case".

#!/bin/sh

hg init
echo This is file a1 > a
hg add a
hg commit -m "commit #0" -d "1000000 0"
touch b
hg add b
rm b
hg commit -A -m"comment #1" -d "1000000 0"