ENH: add git names to test_perf.py
Let users pass nice git names so that they don't have to manually pass sha1 
hashes to test between commits.

with this you can do things like `./test_perf.sh -b upstream/master -t HEAD~2` or whatever `git` allows to specify commits
 :+1: 
 haha...this explains the weird errors I was getting when I tried to do this :+1: 
