gitaly-git2go: Ignore nsecs and timezone in tests
git(1) stores commits with a one second precision, and it stores the timezone offset, but not it's name.
This change sets the nanosecond offset for DefaultAuthor
to zero, and
the timezone name to an empty string. This makes it possible to use
require.Equal
to compare a given git2go.Signature
with a signature
deserialized from an actual commit.
Fixes #3534 (closed).