Benchmark profiles for file text-replacement

Prompted by this StackOverflow question:

http://stackoverflow.com/questions/18647400/git-filter-branch-to-remove-strings-but-where-strings-contain-and-other-c#comment27469603_18651069

...which initally reported the BFG was slower than
git-filter-branch. That was later retracted, but I had a few ideas
about how maybe we could go faster, so gotta get those benchmarks
in first.