log: make file log slow path usable on large repos Running "hg log <pattern or directory>" on large repos took a very, very long time because it first read ctx.files() for every commit before even starting to process the results. This change makes the ctx.files() check lazy, which makes the command start producing results immediately.

plan9: update util.py for cpython 2.7 build

parse_manifest: rewrite to use memchr memchr is usually smarter than a simple for loop. With gcc 4.4.6 and glibc 2.12 on x86-64, for a 20 MB, 200,000 file manifest, parse_manifest goes from 0.116 seconds to 0.095 seconds.