GC: Replace Files methods with File alternatives

The Files.exists method has noticeably poor performance in JDK 8 and can
slow an application significantly when used to check files that do not
actually exist. The same goes for Files.notExists, Files.isDirectory and
Files.isRegularFile [1].

Replace them with their File counterpart.

[1] https://rules.sonarsource.com/java/tag/performance/RSPEC-3725

Signed-off-by: Hector Oswaldo Caballero <hector.caballero@ericsson.com>
Change-Id: I89d23b9cc74bec8e05f6b7f3e49bfd967dbb6373

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

@@ -1525,12 +1525,12 @@ public class GC {
 		int n = 0;
 		int threshold = (auto + 255) / 256;
 		Path dir = repo.getObjectsDirectory().toPath().resolve("17"); //$NON-NLS-1$
-		if (!Files.exists(dir)) {
+		if (!dir.toFile().exists()) {
 			return false;
 		}
 		try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir, file -> {
 					Path fileName = file.getFileName();
-					return Files.isRegularFile(file) && fileName != null
+					return file.toFile().isFile() && fileName != null
 							&& PATTERN_LOOSE_OBJECT.matcher(fileName.toString())
 									.matches();
 				})) {