7b20a6045e8d2a25c86633461c03b13353915643 introduced support for SSH
signatures to the `catfile` package. The GetCommitSignatures RPC uses a
different implementation. This implementation is already compatible with
SSH sigantures, but is not tested with them. This change adds tests
which use SSH signatures.
Changelog: other
For the test repositories in _build/testrepos a fixed list of refs is
defined. This list will ensure tests remain to succeed, even if upstream
new refs are pushed.
This change adds two more refs which will be used in rebase tests in
future commits.
