GetArchive: Support excluding paths

Ethan Reesorrequested to merge
ethan.reesor/contrib/gitaly:2893-archive-exclusions into master

This MR adds repeated bytes exclude = 6 to RepositoryService.GetArchiveRequest. Exclusions will be passed to git archive as :(exclude)<path> pathspecs. This is necessary for Go support for replacing custom archive generation with a call to GetArchive: gitlab#223577 (closed).

Closes #2893 (closed).

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