WIP: Use repository cache without update if offline

This is a very crude way of doing it, but it works.

I think we need to figure out three things to make this prod worthy:
- determine that we failed to update because of missing network connectivity
- check that the cache is "valid" -- ideally this would involve checking whether it can fulfill the dependency requirements, but I don't think that is really possible. We should validate that there's an actual clone present, though
- how to communicate the fact that we chose an offline version of the cache, maybe simply a warning diagnostic?

Author: neonichu

Sources/SourceControl/RepositoryManager.swift

@@ -325,12 +325,21 @@ public class RepositoryManager: Cancellable {
                     }
                 }
             } catch {
-                cacheUsed = false
-                // Fetch without populating the cache in the case of an error.
-                observabilityScope.emit(warning: "skipping cache due to an error: \(error)")
-                // it is possible that we already created the directory from failed attempts, so clear leftover data if present.
-                try? self.fileSystem.removeFileTree(repositoryPath)
-                try self.provider.fetch(repository: handle.repository, to: repositoryPath, progressHandler: updateFetchProgress(progress:))
+                // TODO: Better detection of whether the error was due to be offline
+                if "\(error)".contains("Could not resolve host") {
+                    // TODO: check validity of the cache
+                    cacheUsed = true
+                    // Copy the repository from the cache into the repository path.
+                    try self.fileSystem.createDirectory(repositoryPath.parentDirectory, recursive: true)
+                    try self.provider.copy(from: cachedRepositoryPath, to: repositoryPath)
+                } else {
+                    cacheUsed = false
+                    // Fetch without populating the cache in the case of an error.
+                    observabilityScope.emit(warning: "skipping cache due to an error: \(error)")
+                    // it is possible that we already created the directory from failed attempts, so clear leftover data if present.
+                    try? self.fileSystem.removeFileTree(repositoryPath)
+                    try self.provider.fetch(repository: handle.repository, to: repositoryPath, progressHandler: updateFetchProgress(progress:))
+                }
             }
         } else {
             // it is possible that we already created the directory from failed attempts, so clear leftover data if present.