Change FS not to throw NPE when facing InMemory databases

The FS class and the subclasses FS_POSIX assumed in the findHook()
method that every repository has a valid gitDir. But in tests when using
in-memory-repositories this is not true and this method was generating
NPEs. Change the method to return null if no repository directory can be
determined.

Change-Id: I38a4d36dc6452b5dacae3d0dbf562b569ca3c19b

org.eclipse.jgit/src/org/eclipse/jgit/util/FS.java

@@ -866,7 +866,10 @@ public abstract class FS {
 	 * @since 4.0
 	 */
 	public File findHook(Repository repository, final String hookName) {
-		final File hookFile = new File(new File(repository.getDirectory(),
+		File gitDir = repository.getDirectory();
+		if (gitDir == null)
+			return null;
+		final File hookFile = new File(new File(gitDir,
 				Constants.HOOKS), hookName);
 		return hookFile.isFile() ? hookFile : null;
 	}

org.eclipse.jgit/src/org/eclipse/jgit/util/FS_POSIX.java

@@ -335,6 +335,9 @@ public class FS_POSIX extends FS {
 	@Override
 	public File findHook(Repository repository, String hookName) {
 		final File gitdir = repository.getDirectory();
+		if (gitdir == null) {
+			return null;
+		}
 		final Path hookPath = gitdir.toPath().resolve(Constants.HOOKS)
 				.resolve(hookName);
 		if (Files.isExecutable(hookPath))