Add --prefix argument to archive-source command (#3748)

Author: mattt

Sources/SourceControl/GitRepository.swift

@@ -523,6 +523,7 @@ public final class GitRepository: Repository, WorkingCheckout {
         try self.lock.withLock {
             try callGit("archive",
                         "--format", "zip",
+                        "--prefix", path.basenameWithoutExt,
                         "--output", path.pathString,
                         "HEAD",
                         failureMessage: "Couldn’t create an archive")