[SR-13484] Use --no-checkout when cloning packages, since it's immediately followed by a checkout anyway

Sources/SourceControl/GitRepository.swift

@@ -83,7 +83,7 @@ public class GitRepositoryProvider: RepositoryProvider {
             // a clone from our cache of repositories and then we replace the remote to the one originally
             // present in the bare repository.
             try Process.checkNonZeroExit(args:
-                    Git.tool, "clone", sourcePath.pathString, destinationPath.pathString)
+                    Git.tool, "clone", "--no-checkout", sourcePath.pathString, destinationPath.pathString)
             // The default name of the remote.
             let origin = "origin"
             // In destination repo remove the remote which will be pointing to the source repo.
@@ -102,7 +102,7 @@ public class GitRepositoryProvider: RepositoryProvider {
             // only ever expect to get back a revision that remains present in the
             // object storage.
             try Process.checkNonZeroExit(args:
-                    Git.tool, "clone", "--shared", sourcePath.pathString, destinationPath.pathString)
+                    Git.tool, "clone", "--shared", "--no-checkout", sourcePath.pathString, destinationPath.pathString)
         }
     }
 

Sources/SourceControl/InMemoryGitRepository.swift

@@ -341,7 +341,6 @@ public final class InMemoryGitRepositoryProvider: RepositoryProvider {
     ) throws {
         let checkout = fetchedMap[sourcePath]!.copy(at: destinationPath)
         checkoutsMap[destinationPath] = checkout
-        try checkout.installHead()
     }
 
     public func checkoutExists(at path: AbsolutePath) throws -> Bool {

Sources/SourceControl/Repository.swift

@@ -83,7 +83,10 @@ public protocol RepositoryProvider {
 
     /// Clone a managed repository into a working copy at on the local file system.
     ///
-    /// Once complete, the repository can be opened using `openCheckout`.
+    /// Once complete, the repository can be opened using `openCheckout`. Note
+    /// that there is no requirement that the files have been materialized into
+    /// the file system at the completion of this call, since it will always be
+    /// followed by checking out the cloned working copy at a particular ref.
     ///
     /// - Parameters:
     ///   - sourcePath: The location of the repository on disk, at which the

Tests/SourceControlTests/GitRepositoryTests.swift

@@ -639,4 +639,43 @@ class GitRepositoryTests: XCTestCase {
             XCTAssertTrue(ignored[0])
         }
     }
+
+    func testMissingDefaultBranch() throws {
+        mktmpdir { path in
+            // Create a repository.
+            let testRepoPath = path.appending(component: "test-repo")
+            try makeDirectories(testRepoPath)
+            initGitRepo(testRepoPath)
+            let repo = GitRepository(path: testRepoPath)
+
+            // Create a `main` branch and remove `master`.
+            try repo.checkout(newBranch: "main")
+            try systemQuietly([Git.tool, "-C", testRepoPath.pathString, "branch", "-D", "master"])
+
+            // Change the branch name to something non-existent.
+            try systemQuietly([Git.tool, "-C", testRepoPath.pathString, "symbolic-ref", "HEAD", "refs/heads/_non_existent_branch_"])
+
+            // Clone it somewhere.
+            let testClonePath = path.appending(component: "clone")
+            let provider = GitRepositoryProvider()
+            let repoSpec = RepositorySpecifier(url: testRepoPath.pathString)
+            try provider.fetch(repository: repoSpec, to: testClonePath)
+            let clonedRepo = provider.open(repository: repoSpec, at: testClonePath)
+            XCTAssertEqual(clonedRepo.tags, [])
+
+            // Clone off a checkout.
+            let checkoutPath = path.appending(component: "checkout")
+            try provider.cloneCheckout(repository: repoSpec, at: testClonePath, to: checkoutPath, editable: false)
+            XCTAssertFalse(localFileSystem.exists(checkoutPath.appending(component: "file.swift")))
+            let checkoutRepo = try provider.openCheckout(at: checkoutPath)
+
+            // Try to check out the `main` branch.
+            try checkoutRepo.checkout(revision: Revision(identifier: "main"))
+            XCTAssertTrue(localFileSystem.exists(checkoutPath.appending(component: "file.swift")))
+
+            // The following will throw if HEAD was set incorrectly and we didn't do a no-checkout clone.
+            XCTAssertNoThrow(try checkoutRepo.getCurrentRevision())
+        }
+    }
+
 }