Fix NoSuchFileException in GC.deleteTempPacksIdx()

This exception is thrown in GC.deleteTempPacksIdx() if the repository
has no packs.

Bug: 538286
Change-Id: Ieb482be751226baf0843068a0f847e0cdc6e0cb6
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/file/GC.java

@@ -996,6 +996,9 @@ public class GC {
 	private void deleteTempPacksIdx() {
 		Path packDir = repo.getObjectDatabase().getPackDirectory().toPath();
 		Instant threshold = Instant.now().minus(1, ChronoUnit.DAYS);
+		if (!Files.exists(packDir)) {
+			return;
+		}
 		try (DirectoryStream<Path> stream =
 				Files.newDirectoryStream(packDir, "gc_*_tmp")) { //$NON-NLS-1$
 			stream.forEach(t -> {