commit: Use `ObjectReaderWithoutMailmap` in `GetCommitSignatures`
When a commit is signed, it is either signed by the user or by Gitaly itself. In both the scenarios, the commit author can be different from the output of the 'git-cat-file(1)' used with 'mailmap'. This difference can cause the verification of the signature to fail. Since signature verification is important, let's skip using 'mailmap' in the GetCommitSignatures
RPC.
Closes #6273 (closed)