Disable githoooks for git annex commits

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/40

<s>This fix checks if the commit `SHA` is also in one of the synced `git-annex` branches, which means that it should safely skip git hooks. </s> This may not work with multiple commits to the synced branch

These `git-annex` commits also consistently have a message starting with git-annex blah... Using this, as probability of false positives is very low.

See merge request !272