Also see "Removing a File from Every Commit" http://git-scm.com/book/en/Git-Tools-Rewriting-History

removeFileFromGit.sh

# Warning this is a destructive operation.
git ls-tree --abbrev -r -l HEAD | sort +3 | tail
git filter-branch --all --tree-filter 'rm -f blueray.iso' HEAD

# Git keeps track of the original refs which keeps the repo large.
# We remove those now.

# 1) Make sure original refs are gone:
rm -rf .git/refs/original

# 2) Expire all reflog entries:
git reflog expire --all --expire='0 days'

# 3) Check for old packed refs
cp .git/packed-refs .git/packed-refs~
vi .git/packed-refs
rm -rf .git/refs/original/
git fsck --full --unreachable

# 4) Repack your packed archive(s)
git repack -A -d

# 5) Prune loose (unreachable) objects
git  gc --prune=now