osutil: write a C implementation of statfiles for unix
This makes a big difference to performance.
In a clean working directory containing 170,000 files, performance of
"hg --time diff" improves from 2.38 seconds to 1.69.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown