Add FileRepository(String) convenience constructor

Add a convenience API in FileRepository to pass in a String that
points to the GIT_DIR location.  This is converted to a File and
sent through the usual constructor.

Change-Id: I588388f37e89b8c690020f110a1bc59f46170c40

org.eclipse.jgit/src/org/eclipse/jgit/storage/file/FileRepository.java

@@ -123,6 +123,20 @@ public class FileRepository extends Repository {
 		this(new FileRepositoryBuilder().setGitDir(gitDir).setup());
 	}
 
+	/**
+	 * A convenience API for {@link #FileRepository(File)}.
+	 *
+	 * @param gitDir
+	 *            GIT_DIR (the location of the repository metadata).
+	 * @throws IOException
+	 *             the repository appears to already exist but cannot be
+	 *             accessed.
+	 * @see FileRepositoryBuilder
+	 */
+	public FileRepository(final String gitDir) throws IOException {
+		this(new File(gitDir));
+	}
+
 	/**
 	 * Create a repository using the local file system.
 	 *