store: implement fncache basic path encoding in C (This is not yet enabled; it will be turned on in a followup patch.) The path encoding performed by fncache is complex and (perhaps surprisingly) slow enough to negatively affect the overall performance of Mercurial. For a short path (< 120 bytes), the Python code can be reduced to a fairly tractable state machine that either determines that nothing needs to be done in a single pass, or performs the encoding in a second pass. For longer paths, we avoid the more complicated hashed encoding scheme for now, and fall back to Python. Raw performance: I measured in a repo containing 150,000 files in its tip manifest, with a median path name length of 57 bytes, and 95th percentile of 96 bytes. In this repo, the Python code takes 3.1 seconds to encode all path names, while the hybrid C-and-Python code (called from Python) takes 0.21 seconds, for a speedup of about 14. Across several other large repositories, I've measured the speedup from the C code at between 26x and 40x. For path names above 120 bytes where we must fall back to Python for hashed encoding, the speedup is about 1.7x. Thus absolute performance will depend strongly on the characteristics of a particular repository.