SourceControl: attempt to enable long path support for Windows (#6553)

Windows paths are limited to 261 characters by default.  Git provides
the `core.longpaths` option to use the extended path support.  This
should make SPM more reliable on Windows.

Author: compnerd

Sources/SourceControl/GitRepository.swift

@@ -163,6 +163,9 @@ public struct GitRepositoryProvider: RepositoryProvider, Cancellable {
             "-c", "core.symlinks=true",
             // Disable fsmonitor to avoid spawning a monitor process.
             "-c", "core.fsmonitor=false",
+            // Enable long path support on Windows as otherwise we are limited
+            // to 261 characters in the complete path.
+            "-c", "core.longpaths=true",
         ] + options + [origin, destination]
 
         try self.callGit(