Add some performance testing scripts

Add some performance testing scripts

find_patchbomb.py: Given a git repo, finds the longest linear sequence
  of commits. Useful for testing StGit on a real repository.

setup.sh: Creates two test repositories, one synthetic and one based
  on the Linux kernel repo, with strategically placed tags.

create_synthetic_repo.py: Helper script for setup.sh; it produces
  output that is to be fed to git fast-import.

perftest.py: Runs one of a (small) number of hard-coded performance
  tests against a copy of one of the repos created by setup.sh. The
  initial testcases all involve uncommitting a large number of patches
  and then rebasing them.

Signed-off-by: Karl Hasselström <kha@treskal.com>