dirstate: use parsers.make_file_foldmap when available This is a significant performance win on large repositories. perffilefoldmap: On Linux/gcc, on a test repo with over 500,000 files: before: wall 0.605021 comb 0.600000 user 0.560000 sys 0.040000 (best of 17) after: wall 0.280530 comb 0.280000 user 0.250000 sys 0.030000 (best of 35) On Mac OS X/clang, on a real-world repo with over 200,000 files: before: wall 0.281103 comb 0.280000 user 0.260000 sys 0.020000 (best of 34) after: wall 0.133622 comb 0.140000 user 0.120000 sys 0.020000 (best of 65) This visibly impacts status times on case-insensitive file systems. On the Mac OS X repo, status goes from 3.64 seconds to 3.50. With the third-party hgwatchman extension [1], 'hg status' on the same repo goes from 0.80 seconds to 0.65. [1] https://bitbucket.org/facebook/hgwatchman