pgm: Report not in git repository rather than NPE

If we can't find a Git repository after searching all the way up
to the filesystem root, JGit threw an NPE because we tried to get
the path of null.

Change-Id: I4e42364aeba53993c0ea528a9aeba3f08c7b3321
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Reviewed-by: Robin Rosenberg <robin.rosenberg@dewire.com>

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Main.java

@@ -232,7 +232,8 @@ public class Main {
 			if (gitDir.isDirectory())
 				return gitDir;
 			current = current.getParentFile();
-			if (ceilingDirectories.contains(current.getPath()))
+			if (current != null
+					&& ceilingDirectories.contains(current.getPath()))
 				break;
 		}
 		return null;