log: Capture stderr of git-diff in CommitDiff

When git-diff(1) fails on the cmd.Wait() call within the eachDiff iterator inside the CommiDiff RPC handler, we return a generic error message in the RPC response. The actual error message produced by the child process is avilable in stderr.

Capture stderr and return it to the client as gRPC metadata. This will also ensure it's logged for further analysis.