Fix 'unexpected operator' issue

For some reason when using == for string comparison leads to the following error on my Ubuntu 13.04:

Rewrite 3a5437f0df459d86b66b42892a87bbb262acef72 (1/16)/usr/lib/git-core/git-filter-branch: 1: [: Kaloyan Raev: unexpected operator

Replacing == with = resolved the issue.