Propagate sign param to UserCommitFiles RPC
What does this MR do and why?
Propagate sign param to UserCommitFiles RPC. The param was introduced in Add Sign field to commit UserCommitFiles RPC (gitaly!6795 - merged)
If the param is true and commit signing is enabled, then the resulting commit will be signed. Default value is true because we want the commits to be signed when commit signing is enabled.