We had something similar before building pre-commit and it served us well for many years. The issue is it grew to be a really long bash file with a lot of copy and pasted code (for lots of different hooks). We built pre-commit to reduce the code duplication and allow all developers to install new hooks that are not installed on the system. We also handle interesting cases like dealing with merge conflicts (see "pre-commit during merges" http://pre-commit.com/#advanced).

aside: It would be cool to support mercurial in the future :D